Italian side Benetton Treviso will leave the Pro12 at the end of the season because of uncertainty over its future.
A statement on the club's website said "a prolonged lack of final decisions" had led them not to renew their contract with the tournament.
Clubs from Wales, Ireland, Scotland and Italy participate in the competition.
Treviso say they will consider taking part in a tournament the Italian Rugby Federation intends to promote.
They cite indecision over the "organisation, participation and regulations" of the Pro12 as the main reason for the decision.
Organisers of the competition, which replaced the Celtic League when teams from Italy joined in 2010, are understood to be in negotiations over future Italian participation.
Italian Rugby Federation president Alfredo Gavazzi had said the organisation intends to withdraw Treviso and Zebre from the Pro12 and enter them in an eight-team Italian League.
Treviso have competed in the Pro12 since 2010, and had previously competed in the Italian domestic championship.
Treviso's withdrawal comes amid concern over the future of all European competitions.