The Heineken Champions Cup will be expanded to 24 teams in the 2020/21 season to offset the impact of Covid-19.
Organisers European Professional Club Rugby (EPCR) have agreed to add four teams to the competition – and alter the format for one season to cut one match from the campaign.
The new-look Champions Cup will be split into four pool-stage matches, a two-legged quarter-final, a one-off semi-final and the final in Marseille on May 22, 2021.
The 24 clubs will be split into two pools of 12, and teams from the same country will not face each other in the group stage.
The Premiership, France's Top 14 and the PRO14 will each provide eight teams to the tournament.
"With an expanded knockout stage and no repeats of domestic matches during the pool stage, this format creates brand new competitive opportunities for Europe's elite clubs and their fans," said EPCR chairman Simon Halliday.
On announcing the changes, EPCR confirmed in a statement that the pandemic had forced the switch.
"The decision to alter the formats for next season on an exceptional basis has been made against the backdrop of the COVID-19 public health crisis and its ongoing impact on the professional club game in Europe," read the EPCR statement.
The 2020/21 Challenge Cup will comprise 14 clubs; six from France's Top 14, four from the Premiership and four from the PRO14.
Four rounds of pool matches will be followed by single knockout matches from a round-of-16 onwards.
The Challenge Cup's top eight clubs will progress to the last 16, to be joined by eight teams dropping down from the Champions Cup.
