Britain's biggest teaching union has blocked an agreement on classroom assistants.

The National Union of Teachers was the only teaching union not to sign up to an agreement which would have reduced teachers' workload by increasing the role played by support staff.

A spokesman for the NUT told the Telegraph on Monday that the union did not object to an increase in pay for the classroom assistants, who "play a valuable role supporting teachers".

But it refused to accept the terms of the agreement because it gave assistants a management role.

The GMB union, which represent school support staff, says that 80 per cent of classroom assistants already mark pupils' work, while half deliver lessons and 20 per cent cover for teachers.

The Surrey plans would divide the job of classroom assistant into three grades: team leader; advanced teaching assistant and teaching assistant.

Team leaders would be paid around £17,500 - the same salary of a newly qualified teacher, but without paid holidays.

Ian McNicol, the GMB officer for Surrey, said the guidance had gone out to schools without the top "team leader" category, pending further discussions with the NUT.

"Our members do not want to be teachers on the cheap either," he said.

"What they are looking for is recognition of the roles and responsibilities they are taking on in schools."