Adds analyst comment, updates share price
** Rightmove RMV.L slide nearly 9%, hitting a 6-year low
** Traders point to a BBC report saying estate agents accuse the property portal of charging "unsustainable" fees
** BBC reports Rightmove faces a proposed class action; letter of claim seeks just under 1.5 bln pounds in damages
** Rightmove says aware of reports an application to start collective proceedings has been filed with the Competition Appeal Tribunal
** Company says it is "confident in the value we provide to our partners and consumers, who are at the core of our business"
** RBC calls slump "unwarranted", noting the claim was known and unlikely to succeed: "Rightmove has a dominant position because it is very good at what it does"
** By 1100 GMT, Rightmove off lows, down 5.3%. Stock down over 21% YTD
(Reporting by Danilo Masoni and Samuel Indyk editing by Alun John)
((Danilo.Masoni@tr.com; Reuters Messaging: danilo.masoni.thomsonreuters.com@reuters.net/))