The move means teams will be allowed to select an extra two players for their squads for the Pool stages of each competition, bringing the squad limit from 34 to 36. An extra three players can be selected if a team reaches a tournament's knockout stages.
"Clubs approached ERC and asked us to reconsider squad sizes due to the challenges and length of the tournaments, in particular when it came to rounds five and six," said ERC chief executive Derek McGrath.
"They said injuries were playing an increasing part in denying them fielding their best teams.
"After considering the requests and receiving feedback from the clubs the board of ERC approved an increase in the squad size to 36 players with the stipulation that 10 of them should be front row forwards."
Ulster (home v Treviso) and Munster (away v Sale) kick-off off their Heineken Cup Pool campaigns on Friday, October 21, while Leinster get theirs off the ground on Saturday, October 22, at home to Bath.
Connacht's Challenge Cup run starts with a home tie against Italian side Catania on Saturday, October 22.