Ministers have a month to persuade the biggest support staff union that it is worth remaining in the school workforce agreement.

Unison members voted in June to suspend involvement in the deal and to renegotiate the agreement to include a national pay and career structure for support staff.

The union says there must be “genuine progress” in talks with employers and the Government by September 8, when it decides how to act on conference motions highly critical of the agreement.

Unison’s annual conference voted to threaten industrial action if the Government failed to negotiate over the deal and to withdraw until it becomes compulsory for schools and local authorities to reach agreement with support staff unions on pay and conditions.

Heads have warned that the next stages of the agreement - limits on the time teachers can spend covering for absent colleagues and guaranteed non-contact time for teachers - could be jeopardised in some schools if Unison pulls out.

Since June, Unison’s leadership, already frustrated at the slow progress towards better support staff pay and conditions, has been alarmed by the Government’s five-year education strategy, which smooths the way for all secondary schools to get foundation status. Foundation schools act as individual employers and having to negotiate with each of these would make Unison’s goals even harder to achieve.

Christina McAnea, Unison head of education, said: “Progress has been incredibly slow and what we don’t want to see is further fragmentation of pay and conditions down to school level.

“The Government is prepared to listen to what we have got to say but its five-year plan is all about giving schools additional freedoms when we want better pay and conditions for support staff introduced in a structured way.”

The fact that support staff do not benefit from the same national pay structure as teachers has proved one of Unison’s biggest obstacles in selling the the workforce agreement to its members.

In June, two-and-a-half terms into the agreement, Unison reported that nearly half of local authorities had still not begun negotiating new career structures for support staff.

Ms McAnea said that even where structures were in place some schools were ignoring them. Pay for cover supervisors varied from pound;11,000 to pound;18,000, depending on where they worked.

“At the moment schools are almost plucking figures from (the air) in terms of pay,” she said. “What we need is a national structure.”

Currently there are only nationally agreed “job profiles”, which can be matched to locally agreed grades. Unison wants national grades, with guideline pay for each. But Ms McAnea said there had been “fruitful” negotiations with employers about strengthening guidance on support staff employment by September.