The UEFA football president Michel Platini says having 24 teams at European championship finals instead of 16 from 2016 will not affect the quality of matches.
The expansion has been approved unanimously.
It was expected, despite criticism that it could dilute the quality of the event, after senior officials of all 53 UEFA member nations had backed the idea in June.
The format will consist of six groups of four teams.
The top two from each group will qualify and will be joined in the round of 16 by the four best third-placed sides.
There will be 51 matches, compared with 31 now, played over 29 to 31 days, depending on the match schedule.
The traditional qualifying format, with groups of six and five teams, will stay in place.