England's tour to North America has been put in jeopardy after talks broke down between club owners and the Rugby Football Union.
Talks between Premier Rugby Partnership and the RFU had continued throughout yesterday to try and to reach a resolution over the future of the sport.
Chief executive of the Professional Rugby Players' Association Damien Hopley has warned squad members the five-match tour, due to start on Monday, could be called off.
"The players want to go but I have had to tell them to be prepared to stand down," Hopley told The Times.
"The international between England and the Barbarians on Sunday will go ahead as scheduled and the dispute will not affect the participation of the 18 Englishmen selected for the British Isles tour to Australia."
In March the clubs had said they would consider the withdrawing services as a last resort if talks continued to stall.
Cancellation of the tour would lead to severe censure by the International Rugby Board as well as the payment of hefty compensation to the US and Canadian unions.