The National Union of Teachers has threatened strike action over failure to reach agreement on teacher workloads.

Although teaching staff should no longer carry out routine administrative duties, heads have argued that they cannot afford the support staff.

The union's general secretary also warned that his members' workloads could increase by having to mark lesson plans by unqualified colleagues, while covering for staff who are made redundant and not replaced.

Although the NUT was the only union not to sign up to the agreement with ministers, Doug McAvoy argued that his members should still benefit from it.

"The government promised teachers a reduced workload," he said. "They promised parents ever-improving provision of education.

"At the moment there must be serious doubts that either group will be satisfied with what the government, with the other teacher organisations, is doing."

The Department for Education and Skills claimed industrial action would "achieve nothing other than the disruption of education for children".

"We are working with all the unions representing the vast majority of teachers and support staff over the next few months and are determined to ensure that the first phase of the reforms are delivered by the school team working together," said a spokesman.

The National Association of Head Teachers criticised the NUT for the threat.

"I think it is hypocrisy of the worst order for people who refused to sign the agreement to threaten industrial action when head teachers are accused of refusing to implement the very agreement that the NUT has been attacking morning, noon and night for the last nine months," said leader David Hart.

"Threatening industrial action might seem very macho in the eyes of their members but it doesn't make any contribution whatsoever to solving the problems of those schools which will be struggling for genuine reasons.

"There isn't a single head in this country who doesn't want to reduce the workload of their staff but at the end of the day we have to get round the table in order to help those schools that are in real difficulty."