European championship finals will feature 24 teams instead of 16 from 2016, senior UEFA committee member Franz Beckenbauer said on Thursday.
"It has been agreed, it will be announced at a news conference tomorrow," Beckenbauer told Reuters.
The former Germany World Cup-winning captain and coach, who is also a member of FIFA's executive committee, said the expansion was endorsed on the first day of a two-day meeting of the executive committee of European soccer's governing body.
The decision was expected, despite criticism it could dilute the quality of the event, after senior officials of all 53 UEFA member nations backed the idea in June.
Poland and Ukraine's lack of progress in preparing to host Euro 2012 was also on the agenda.
UEFA was expected to stand by its decision to choose the two countries as hosts, with a final decision expected on Friday.