European football's governing body had already announced in April that the competition would expand from 32 teams to 36 in two seasons' time.
And following talks in Vienna on Wednesday (AEST), the UEFA Executive Committee confirmed the number of rounds in the group stage will increase from six to eight.
All group and knockout-stage games up until the final will continue to be staged on midweek days, as it currently the case.
Two of the four additional places in the expanded format will be awarded on the basis of the highest-performing countries from the past season across UEFA club competitions.
If that had been the case this season, an additional team from the Premier League and Eredivisie would have qualified for next season's tournament.
It had previously been reported that those two places would go to clubs on the basis of their historic performance in European competition, but that is no longer the case.
Of the other two spots, an extra team will qualify from the fifth-ranked country in Europe, while another will go to one of the domestic champions who do not qualify automatically.
Commenting on the changes, UEFA president Aleksander Ceferin, said: "UEFA has clearly shown today that we are fully committed to respecting the fundamental values of sport and to defending the key principle of open competitions, with qualification based on sporting merit, fully in line with the values and solidarity-based European sports model.
"Today's decisions conclude an extensive consultation process during which we listened to the ideas of fans, players, coaches, national associations, clubs and leagues to name but a few, with the aim to find the best solution for the development and success of European football, both domestically and on the international club stage."
Under the new format, the initial phase will contain a single league consisting of all 36 teams, with each side playing four home games and four away games against eight different opponents.
The top eight sides in the league will qualify automatically for the knockout stage, while the teams finishing in ninth to 24th place will compete in a two-legged play-off to secure their path to the last 16.
Ceferin added: "We are convinced that the format chosen strikes the right balance and that it will improve the competitive balance and generate solid revenues that can be distributed to clubs, leagues and into grassroots football across our continent while increasing the appeal and popularity of our club competitions.
"I am really pleased that it was a unanimous decision of the UEFA Executive Committee, with the European Club Association, European Leagues and National Associations all agreeing with the proposal made. Another proof that European football is more united than ever.
"Qualification will thus remain purely based on sporting performance and the dream to participate will remain for all clubs."
Similar format changes will also be applied to the Europa League and Europa Conference League, with both also including 36 teams in the initial league phase.