Schools which stay open during tomorrow’s teachers’ strike by asking teaching assistants to cover for their absent colleagues could face further walk-outs, unions have warned.

Thousands of teachers in the NUT and NASUWT unions working in schools in London, the South West, South East, North East and Cumbria will taking part in the one-day strike, as part of the unions’ ongoing campaign of industrial action over pay, pensions and working conditions.

Earlier this month, TES reported that a survey by ATL - the only major classroom union not to take part in the strike - found that one in three school support staff said they were asked to take classes themselves on a regular basis. ATL general secretary Mary Bousted said schools were guilty of the “exploitation of support staff who are being used as a cheap option to teachers”.

While less than a third (29 per cent) of schools were closed by the NUT and NASUWT’s first regional strike earlier this month, the unions are concerned that some heads could ask support staff to teach in order to stay open through tomorrow’s action.

“We have received reports of this,” said NASUWT general secretary Chris Keates. “All of the reports we have seen where this has happened we have then informed Unison and GMB [about]. We think it is completely inappropriate that head teachers are putting support staff in this position.”

John Dixon, head of the NUT’s membership department, said teaching assistants were “strongly advised to refuse any such requests”. “If this practice goes ahead, it would almost certainly lead to a breakdown in relationships between teachers and employer, and could lead to further action,” he said.

Jon Richards, national secretary for education and children’s services at public sector union Unison, which represents 247,000 members working in schools, said it had received a small number of calls from members concerned about work they were asked to undertake during the 1 October strike.

A letter sent to its members ahead of the first strike stressed that support staff “should not be moved from the duties they would normally have carried out in order to cover classes and frustrate the industrial action of colleagues”.

The NUT and NASUWT are also planning a national walk-out before the end of term, but the date has not yet been announced.