The Education Secretary, Michael Gove, has written to schools to advise headteachers that they could dock the pay of NUT and NASUWT members of the taking part in action short of a strike, following advice from Government lawyers.
Mr Gove said work-to-rule 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.
In his letter, he said: "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.
"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".
More top news
The Foreign Secretary described Jeremy Corbyn as a "mugwump" - but what does that obscure word mean?
Readers are shunning e-books in favour of their traditional counterparts, which sold last year at their highest level since 2012.
Glaucoma is the biggest cause of permanent blindness and affects 60 million people around the world - a tenth of whom are completely blind.