The Champions Cup will change its format next season for the third time in five campaigns, the tournament's organiser European Professional Club Rugby (EPCR) announced on Wednesday.
After three terms with two pools of 12 teams, which replaced the long-standing format of five groups of four in 2020, there will be a stage with four groups of six teams.
Clubs from the United Rugby Championship, the French Top 14 and the English Premiership will also participate in the second-tier Challenge Cup alongside two invitational teams.
"There will be no matches between clubs from the same league, so for the purposes of creating the fixtures, each club will play four matches against four different clubs who are not from the same league either home or away during the pool stage," EPCR said in a statement.
"At the conclusion of the pool stage, the four highest-ranked clubs from each pool will qualify for the Round of 16 and the clubs ranked No 5 in each of the pools will qualify for the knockout stage of the EPCR Challenge Cup," it added.
The first round of both competitions will begin between December 8-10 and the finals will take place on the weekend of May 24-25 in London's Tottenham Hotspur Stadium.