A government bid to avert industrial unrest in primary schools has been hit by official research finding that classroom assistants are not easing the workloads of overstretched teachers.

A pledge to recruit an extra 20,000 assistants formed a key plank of Labour's manifesto commitment to cut teacher shortages during the last general election.

Education secretary Estelle Morris has attempted to head off the threat of industrial action over teacher workloads by assuring staff that the assistants would take on some of the burden.

But an Ofsted report published on Tuesday found that assistants help improve basic educational standards but can actually add to the workload of primary school teachers.

Traditionally, assistants carried out menial tasks such as photocopying worksheets, preparing materials for lessons and other administrative jobs.

But following government pressure, they have more recently become involved with helping children with their lessons - a situation that could lead to conflicting demand on teachers, the inspectors said.

"Schools will need to manage these competing priorities carefully to ensure that teachers' workloads do not increase at a time when strenuous efforts are being made nationally to reduce them," said the report.

Shadow education secretary Damian Green warned that the government must avoid trying to use assistants as "substitute teachers to disguise the crisis in teacher retention".

"We need a clear definition of the role of classroom assistants, so that we do not get the ridiculous situation which Ofsted points up, that in some classrooms you have the assistant teaching and the teacher assisting," he said.

His alert was echoed by the general secretary of the National Union of Teachers, Doug McAvoy, who is threatening industrial action unless the government reduces the pressure on teachers.

"The help provided by teaching assistants in the classroom comes at the expense of easing the bureaucratic burdens on teachers," he said. "Assistants play a valuable role and should be properly paid but they cannot be a substitute for teachers."

The number of assistants has risen from 61,000 to 96,000 since Labour came to office.

This number is set to increase further as the government focuses on its promise to cut teachers' workload.

McAvoy believes the policy is failing and must be revised in light of the report.

"It is crystal clear from the report that teaching assistants are not a magical solution for reducing teacher workload," he said.

"The report sends an emphatic message to the chancellor that the growing number of assistants in our schools has not led to a reduction in teachers' workload.

"He still has the task of funding a reduction in teacher workload in his comprehensive spending review."

In a further blow to the government, Ofsted has distanced itself from the use of classroom assistants as a solution to the problem of teacher workload.

The chief inspector of schools, Mike Tomlinson, acknowledged that the research proved that trained teaching assistants were bringing "benefits" but dismissed suggestions that they were reducing pressure on primary teachers.

"But, as yet, the considerable public investment in expanding their numbers has not led to a reduction in teachers' workloads," he said.

The DfES defended the policy saying it showed "potential" to cut teacher workload while boosting standards.

"This report shows well-trained and well-managed teaching assistants help schools raise standards - particularly in literacy and numeracy," said a spokesman.

"We can also see that teaching assistants, used to support the teacher in creative ways, have the potential to help to reduce teacher workload.

"That is why we are working with unions and other bodies to raise the profile of teaching assistants by increasing their numbers and improving their opportunities for training and career development."