WARSAW, June 27 (Reuters) - Polish top utility PGE's PGE.WA 11.6 billion zloty ($3.5 billion) project to build new power plants in Opole will start this summer, Polish Prime Minister Donald Tusk said on Thursday. "This is a very important step, we have been trying for this investment to happen for many months now," Tusk said during his visit to the southern city of Opole. "I am deeply convinced that thanks to today's agreement this investment will start this summer." PGE signed an agreement with the also state-controlled coal miner Kompania Weglowa to cooperate in Opole - Poland's largest energy project to build two new coal-fired power plants. ($1 = 3.3413 Polish zlotys) (Reporting by Agnieszka Barteczko, writing by Adrian Krajewski, editing by Karolina Slowikowska) ((adrian.krajewski@thomsonreuters.com)(+48 22 653 97 09)(Reuters Messaging: adrian.krajewski.thomsonreuters.com@reuters.net)) Keywords: POLAND PGE/