Teams will be allowed to select a 26-player squad instead of the usual 23 at this summer's European Championship.
The change has been implemented to lessen the load on players after a club season compressed because of the coronavirus pandemic.
It will also help managers adapt if outbreaks occur during the tournament.
The decision, taken by Uefa's national teams committee in April, was approved by Uefa's executive committee on Tuesday.
In a statement, it said the rules were approved "in order to guarantee the smooth running and continuity of the competition in light of the Covid-19 pandemic".
A rule allowing each team to make up to five substitutes per game had already been confirmed.
However, 23 players will remain the maximum number permitted on team sheets for matches.
The tournament, which had been scheduled to take place last summer, begins on 11 June. The deadline for submitting squads is 1 June.
England manager Gareth Southgate is expected to announce his squad during the week commencing 24 May.
Steve Clarke, the Scotland manager, had been expected to name a provisional squad in mid-May, but may now miss that step out before making his final selection at the end of the month.
Robert Page will have the job of picking the Wales squad, after confirmation that he will take charge of the team for the tournament.