How the UK's stock markets compare: the Main Market, AIM and Aquis
Every company whose shares you can buy in the UK has a home market, and that market sets the rules it lives by. For ordinary company shares there are three: the London Stock Exchange's Main Market, its junior market AIM, and the much smaller Aquis Growth Market, run by a separate exchange. They exist side by side because one set of rules can't suit both a £100bn bank and a £10m start-up. The top market asks the most of its companies and draws the biggest investors. The junior markets ask less, so that smaller companies can afford their shares to be publicly traded at all. This page explains what each market is for, why the rules differ, and why companies move between the markets or leave them.
Listing rules are a compromise between protecting investors and what companies can afford
A stock market lets companies raise money from the public, and lets investors buy and sell the shares afterwards. Both depend on trust, so every market sets rules for its listed companies: how established they must be to join, what they must publish and how often, and who checks them.
Every listing rule has a cost. Lawyers, auditors, advisers and the directors' time all have to be paid for, and the bill doesn't shrink much with the size of the company. A FTSE 100 company hardly notices it. For a company worth £10m, it's a real burden.
So the UK runs its markets in tiers: a main market with the heaviest rules for large companies, and junior markets with lighter ones for smaller companies.
The lighter the rules, the more of the checking falls on investors themselves.
The tiers have different rule-writers too.
A Main Market company is "listed": its shares are on the Official List kept by the FCA, the City regulator, which writes the UK Listing Rules companies are obliged to follow.
AIM and Aquis companies are "admitted to trading" by the exchange that runs the market, and that exchange writes and enforces their rules itself, under the FCA's supervision.
"AIM-listed" is everyday shorthand, but strictly only Main Market shares are “listed”. The difference decides which funds can own the shares and how they're taxed.
One thing doesn't change from market to market. Every company must announce price-sensitive news as soon as possible, under the same law, as our guide to UK trading statements explains.
The Main Market is for larger companies that can meet the strictest rules
The Main Market is the London Stock Exchange's top tier, home to the FTSE 100 and to most of the companies you'd recognise by name. Most trading companies sit in its "commercial companies" category, and investment trusts have a category of their own.
The bar to join is high. A company must normally be worth at least £30m, with at least 10% of its shares in public hands so that there are enough to trade, and must publish a prospectus approved by the FCA. Once it's in, it must publish its annual results within four months of its year end. It must also follow the UK Corporate Governance Code, the standard for independent directors, board committees and pay, or explain where it doesn't.
Why accept all that? Because the Main Market is where the biggest pools of money are. Only its companies can join the FTSE 100, 250, SmallCap and All-Share, and the index funds that track them buy every company in the index, whatever they think of it. An index place brings buyers a company never had to win over one by one.
AIM exists to help smaller companies afford to go public
The London Stock Exchange opened the “Alternative Investment Market” (AIM) on 19 June 1995 with ten companies. In its rulebook's words, it is "a market for smaller and growing companies". It grew to 1,694 companies at the end of 2007, and had 598, worth £60.5bn, at the end of July 2026. That fall counts takeovers and moves up to the main market as well as failures. At the end of 2024, half of its companies were worth less than £25m.
Its rules follow from the original purpose. There's no minimum size and no minimum number of shares in public hands, so a young company can join long before it would qualify for the Main Market. Annual results are due within six months rather than four, and since August 2026 AIM companies haven't had to sign up to a governance code. Each of those makes being public cheaper for a small company.
In place of the FCA's checks, every AIM company must keep a nominated adviser, or Nomad. It is a firm approved by the Exchange that judges whether the company is fit to join, then advises it on the rules for as long as it stays. Its duties are "owed solely to the Exchange", not to shareholders. If a company loses its Nomad and doesn't find another within six weeks, its shares are cancelled from AIM without a shareholder vote. That happened to 17 companies in 2024.
The Exchange is open about the trade-off. Every AIM admission document must carry a warning in bold that AIM is "designed primarily for emerging and / or growing companies to which a higher investment risk tends to be attached". Since August 2026 the rulebook has called AIM "a buyer beware market". The Exchange checks that companies follow its rules. Whether the business is any good is for investors to judge.
AIM's reputation is half deserved
AIM is known for failures and the occasional fraud, and some of that is earned. Light rules let in companies that are, as one of our earlier guides to AIM put it, "unproven, unprofitable and rely on appealing stories to attract investors". A small, loss-making company is risky wherever it's quoted. AIM simply has more of them.
Its index shows the result. In the five years to 30 September 2026, the FTSE AIM All-Share lost 30.9%, dividends included, while the FTSE All-Share gained 68.2%. Part of that is the long tail of speculative companies. Part is tax: AIM was, as we wrote in 2025, "particularly badly hit" when the cut to its inheritance-tax relief was announced in the 2024 Budget. And part is how the index works. AIM's best companies tend to grow out of it and move up to the Main Market, taking their future gains with them.
So AIM as a whole has been a poor place to be, while some of its companies have been among the UK's best investments. Jet2, AIM's most traded share in July 2026, was the multibagger, as Dart Group, in our guide to "ahead of expectations". About a third of the shares our rules-based NAPS strategy held over its 11 years were on AIM. The work of investing on AIM is finding the best companies and avoiding the worst.
Aquis is smaller again, and run by a different exchange
The Aquis Growth Market belongs to Aquis Stock Exchange, which isn't part of the London Stock Exchange; the Swiss exchange group SIX has owned it since July 2025. The exchange has changed hands many times, having been known as OFEX, ISDX and NEX Exchange over the years before becoming Aquis. It sits below AIM, for smaller companies again. At the end of September 2026 it had 73, worth £983m in all, and the typical company was valued at about £5.7m.
Its two tiers are built for two stages of growth:
Access is "aimed at companies that are at an earlier stage in their growth", and a company needs to be worth only £2m to join.
Apex, for "larger, more established growth companies", asks for £10m, a quarter of the shares in public hands and two years of audited accounts. A company needs an adviser to join, but since January 2026 it can then swap the adviser for a service arranged by Aquis.
Lighter isn't weaker everywhere, though. Apex companies must follow a governance code "as far as possible", which AIM no longer asks. And an Aquis company that wants to leave the market needs 75% of the votes cast and, where one shareholder owns 30% or more, a majority of the other shareholders too. AIM asks only for the 75%.
The differences that matter to an investor are trading, buyers and tax
Day to day, three things that follow from the rules matter more than the rules themselves: how easily the shares trade, who buys them, and how they're taxed.
Smaller shares are harder to trade, and that costs you
Liquidity is how easily you can buy or sell a share without moving its price, and it falls fast down the markets. Smaller companies have fewer shares in public hands, often because founders and directors still own much of the company. Fewer investors follow them, so fewer want to trade on any given day.
It shows in how the shares are traded. Shares in the FTSE indices and the busiest AIM shares trade on the Exchange's order book, where buyers and sellers meet continuously all day. A less-traded share wouldn't draw enough orders for that. So the Exchange gathers its orders into four auctions a day, and for many such shares market makers, firms that stand ready to buy or sell, quote prices in between.
In July 2026 the ten most-traded AIM shares took 42% of all the money traded on AIM, and more than half of AIM's shares traded less than £1m in the whole month. On Aquis in September 2026, 15 of the 73 companies didn't trade at all.
Thin trading costs you twice.
The first cost is the spread: the gap between the price you can buy at and the price you can sell at. A market maker who may have to hold a share for days before anyone else wants it charges more for the risk. Writing in 2023 about small companies no analyst covers, we warned that spreads of "5% upwards" were "not uncommon": "Even if the shares rise, you may still lose money when you sell".
The second is size: a quoted price holds only for a certain number of shares. Buy more and you work through the sellers one price at a time, paying a little more for each lot, and selling works the same way in reverse. A small order hardly notices. A large one pushes the price against itself and can take days to fill.
Big funds often stay away, and that gives private investors an edge
Funds that track the FTSE 100, 250 and All-Share can't own an AIM or Aquis share, because those indices take only Main Market shares. For other funds it's a choice. Most pension and fund rules allow AIM shares. But respondents to the London Stock Exchange's 2025 consultation said fund managers' own liquidity rules, which make sure they can sell fast enough to repay their investors, have pushed them towards larger companies since 2019. Most big investors could buy AIM shares. They choose not to.
Size is the reason. A fund managing hundreds of millions of pounds that wants even 1% of its money in one company needs millions of pounds' worth of its shares. In a share that trades less than £1m a month, that is months of the whole market's trading, bought at a rising price, with the same problem waiting when the fund wants out. Big investors, as our StockRanks book puts it, "are like whales moving through the water".
A private investor isn't a whale. You can build a small holding in a morning, where a fund would need weeks, and sell it as quickly. And because few funds and analysts follow these companies, less of what can be known about them is already in the price. The slow drift after good news in our "ahead of expectations" research is strongest in smaller, less-traded shares. Small companies are where a private investor who does the work has an edge over the funds, because they're the part of the market the funds can't easily reach. The spread and the time it takes to sell are the price of that edge.
The junior markets' tax breaks come from counting as unlisted
Shares on all three markets can go in a stocks and shares ISA. Beyond that, the tax follows the line between listed and admitted.
Stamp duty. Main Market purchases pay 0.5%, though companies first listed since 27 November 2025 are spared it on electronic purchases for three years. AIM and Aquis shares have paid none since 2014, as long as they aren't listed anywhere else. The exemption was written for markets of smaller companies: the main test a market has to pass is that most of its companies are worth less than £450m.
Inheritance tax. Business Relief cuts inheritance tax on business property, including shares in private companies. Because AIM and Aquis shares aren't on the Official List, the tax rules count them as "unquoted", like a private company's shares, so qualifying holdings kept for two years get the relief. Until April 2026 the relief was 100%, which made AIM shares a popular way to plan for inheritance tax. It is now 50%, an effective rate of 20% on HMRC's own example, and an ordinary Main Market holding gets none. For Aquis, the ISA and the relief are our reading of the law, because HMRC's guidance names AIM but not Aquis. GOV.UK sets out what qualifies.
A tax break is a reason to prefer one sound company over another. It isn't a reason to own a weak one. Mark Simpson, who co-writes our daily report, made the point before the 2024 Budget: "none of these rumoured tax changes will affect the fundamental value of listed businesses".
Companies move up the markets as they grow, and some step down to cut costs
A company isn't tied to the market it joined. Moving up from AIM to the Main Market is common: on the London Stock Exchange's own lists, 17 companies did it between January 2024 and July 2026, among them Gamma Communications, CVS Group and Young's, the pub group.
They move to access larger funds. Once a company is big enough, the Main Market's costs are worth paying for the index funds and larger investors it opens up. Volex said its move in July 2026 "will broaden our appeal to a wider group of UK and international investors". It joined the FTSE 250 within about two months, as Johnson Service Group had in 2025. Aquis works as a stepping stone in the same way. RentGuarantor moved up to AIM in 2025, citing "shareholders' requests to improve share liquidity", and The Smarter Web Company went straight from Aquis to the Main Market in February 2026.
Moving down is rarer, and it is about cost. When Creightons, now Potter & Moore, left the Main Market for AIM in 2025, it said the Main Market's "cost and regulatory requirements" had "become progressively higher in recent years". For smaller companies, it said, "AIM provides a more suitable market and environment". A company that has stayed small, with no prospect of an index place, pays the top market's costs and gets little back.
The shares carry on trading without a break, because the company joins the new market as it leaves the old one. Moving up doesn't always need your vote: Mortgage Advice Bureau's move in 2026 didn't, and Aquis's rules need none for a move to a market "of equivalent regulatory standing". You lose no protection, so there's nothing to consent to. Moving down takes some away, so it needs a vote: Creightons needed 75% of the votes cast. Moving up changes your tax as well. The holding stops qualifying for Business Relief, and further purchases can carry stamp duty.
A company can also leave the markets, and how it leaves decides what you're left with
Sometimes a company leaves the stock market altogether. There are three ways out, and they end very differently for shareholders.
Taken over. A buyer, often a rival or a private-equity firm, buys every share. You're paid in cash or in the buyer's shares, and the company's shares are cancelled. This is usually what people mean by a company being "taken private": the whole business passes to a private owner. Our guide to takeover bids explains how it works.
Delisted, with you still a shareholder. A company can also leave without a buyer. It needs 75% of the votes cast at a shareholder meeting and, on the Main Market and Aquis, a majority of the independent shareholders too where one holder controls the company. Nobody buys you out. The company carries on, and can even keep "plc" in its name, and you keep your shares and your rights as a shareholder. What goes is the market: there's no quoted price and no easy way to sell. Some companies arrange a matched-bargain facility, run by firms such as JP Jenkins and Asset Match, which pairs the occasional buyer with the occasional seller. It keeps a door open, but a narrow one.
Forced off. A company can also lose its place without choosing to, as when an AIM company runs out of time to replace its Nomad. Its shareholders end up where they would after a delisting, without having voted for it. As our guide to AIM put it in 2017, a forced cancellation "leaves few options but to go along with the company's terms".
So being taken private and being delisted are different things. In a takeover you're paid and you're out. After a delisting you're still in, but in a company whose shares no longer trade anywhere.
Put together, the three markets work as a ladder. Companies join where their size and budget fit, climb as they grow, and step down or leave when the costs stop paying. Each rung down trades lighter rules for fewer buyers and thinner trading, and the tax follows the rung.
From the Daily Stock Market Report. Every weekday morning Graham, Roland, Mark and the team cover the news from companies across the Main Market, AIM and Aquis, from the FTSE 100 to the smallest junior-market shares. Read today's report · Sign up to our newsletters
Questions readers ask
Does the UK have only three stock markets?
For ordinary company shares, these are the three that matter. Around them sit a few corners: a part of the Main Market for specialist funds sold to professional investors, a small listed market run by Aquis, and PISCES, a trial running until June 2030 that lets private companies' shares change hands in occasional auctions. Venues such as Cboe and Turquoise also trade London shares, but they're only places to trade. The rules and the tax still follow the share's home market.
Is the Alternative Investment Market (AIM) regulated?
Yes. The London Stock Exchange writes and enforces AIM's rules, the FCA supervises the Exchange, and the law on announcing news and on insider dealing applies just as it does on the Main Market. What AIM companies don't follow is the FCA's Listing Rules.
Is Aquis part of the London Stock Exchange?
No. It's a separate exchange, owned by the Swiss group SIX since July 2025. Its companies' announcements go out through the same news services as those of London Stock Exchange companies.
Where to go next
More in this series: How to read a UK trading statement · What to do when a company beats expectations · What to do on a profit warning · What to do on a takeover bid · How to interpret directors' dealings · The Ultimate Guide to Reading Company Announcements