Ambulance workers are being balloted on strike action after their union was de-recognised in a row over patient safety.
The ballot was launched after YAS announced it no longer recognised Unite for negotiations on behalf of its members.
Unite claimed patient safety was being put at risk by plans which will see semi-skilled care assistants sent to 999 call-outs.
Unions said last month that the salaries and skills of more than 300 emergency staff would be downgraded to help save £46m over five years.
Bosses at YAS insisted safety would not be compromised.
They said that with effect from February 4, YAS had decided to cease voluntary recognition of Unite for the purposes of collected bargaining.
Unite’s Terry Cunliffe said bosses were now refusing to talk to the union after it offered a meeting with Acas.
Mr Cunliffe said: “I have to say I have little hope, unfortunately, that we will be in a position where we are not taking action.
“I can’t see, given the trust’s position, that there’s any way of avoiding industrial action.”
Ballot papers were sent out yesterday, and industrial action could be called as early as March 25, Mr Cunliffe said.
YAS said its working relationship with Unite had been “deeply disappointing over a prolonged period”.
Deputy chief executive Stephen Moir said: “We have not received a constructive contribution to the difficult decisions that the Trust has been required to make for the future, particularly as we seek to maintain high-quality care for patients against the realities of the tough economic climate.”