The Government has sparked anger from teachers' leaders after writing to schools warning that union members taking industrial action are "highly likely" to be in breach of their contract.
Head teachers could deduct the pay of members of the National Union of Teachers and NASUWT taking part in action short of a strike, following advice from Government lawyers, said Education Secretary Michael Gove.
In a letter to schools, Mr Gove said action which has been taking place since October over a range of issues including pay, pensions, job losses and workload, was threatening the quality of pupils' education.
"The vast majority of schools are currently unaffected. A small number, however, are starting to see a severe impact and where this is the case, I believe a robust response is needed. I respect the right of teachers to take industrial action, but this action short of a strike lacks a clear purpose or even a set of coherent aims.
"Feedback from parents suggests they find it difficult to understand why the education of their children needs to suffer. I believe we should tackle this action swiftly and firmly before it causes any more damage in schools.
"The legal position is clear: teachers who are following this industrial action are very likely to be in breach of their contracts. Pay deductions represent a lawful response, and the advice sets out how deductions can be made in a proportionate and reasonable way," said the letter.
Chris Keates, General Secretary of the NASUWT, said: "The Secretary of State is recklessly encouraging schools to take punitive action against teachers on the basis of advice which is littered with caveats and ambiguities and which demonstrates quite clearly that the Secretary of State is unable to state categorically that any action being taken by NASUWT and NUT members is in breach of contract. In the light of this, any school which acts on his advice leaves itself vulnerable to extensive and expensive litigation and escalation of industrial action."
Russell Hobby, general secretary of the National Association of Head Teachers, said: "We understand the position of our colleagues in the teaching unions but our duty to pupils overrides all political or industrial considerations."
Christine Blower, general secretary of the National Union of Teachers, said: "This is a continuation of Michael Gove's war on teachers. One might well ask why Michael Gove is releasing this advice now, three months into our action. We believe it is to distract from the GCSE re-grading court case, the isolation that Michael Gove finds himself in over the EBacc and his forced academies policy.
"Our joint action with the NASUWT is a measured response to the unacceptable attacks on teachers. This action short of strike action is not disruptive to pupils' learning but a challenge to the increasing burden of unnecessary tasks and the undermining of acceptable pay and conditions of service imposed by the Secretary of State."