British and Irish Lions captain Sam Warburton is among the Pro12 stars whose contracts expire at the end of the season but are yet to be offered new terms because clubs do not know how big their 2013-14 budgets will be.
Welsh, Irish and Scottish clubs are stalling on deals while they wait for a decision over whether the terms of the Heineken Cup remain the same, are amended or completely supplanted by a new competition.
English and French teams have drawn up plans for a new competition, to be called the Rugby Champions Cup, having failed to reach agreement with Heineken Cup chiefs over qualification and prize money distribution.
Celtic clubs would face a smaller cut of cash in any new tournament or, if forbidden from taking part by their unions, no revenue at all.
Premiership Rugby already has a broadcast deal in place for a successor to the Heineken Cup, despite the competition itself still being uncertain. BT Sport bought rights for European games involving English clubs from next season, as part of a £152m four-year contract which started this term.