Support staff, who say they are being "pushed and bullied" into teaching whole classes, are poised to vote to pull out of the workforce agreement.
Unison's continuing participation in the reforms rests on a knife-edge as it meets for its annual conference in Bournemouth on Monday.
Delegates from the union, representing the vast majority of support staff, will discuss motions calling for it to renegotiate the agreement or suspend its involvement altogether. Their national leadership want to stay on board but Christina McAnea, Unison head of education, said: "It is going to be a tough debate and I am not confident of the outcome."
Paul Elliott, the union's head of local government in Wales, said the agreement had brought benefits for members and progress was being made, though less than in England.
"We recognise the agreement as a positive step forward. It may not be everything we require but at least it gives recognition to the forgotten army of school support staff. We will be seeking additional monies in the next financial year to ensure the agreement is implemented in the right spirit."
Grassroots anger at the lack of benefits for support staff from the agreement is evident in the wording of conference motions.
They claim the deal has been "largely negative and counterproductive", that heads and governors in many areas are "driving a coach and horses" through it, and threaten industrial action if the London government fails to negotiate national terms and conditions for support staff.
Ms McAnea said the deal was starting to deliver benefits and that 20 per cent of local authorities had negotiated new support staff pay and career structures. But she admitted that two-and-a-half terms into the agreement nearly half had not begun the process.
Withdrawal would represent a huge blow to one of the Government's flagship education policies. Last year Unison narrowly voted to remain party to the agreement on the basis that the deal should be given time to work.
It was signed in January 2003 by unions, government and employers with the aim of reducing workload and improving support staff pay and conditions.
But the National Union of Teachers and UCAC, the Welsh-medium teachers'
union, never signed up - and the National Association of Head Teachers is threatening to withdraw if more cash is not made available.