Teachers often spend long periods covering for sick colleagues

Schools minister David Miliband said this would allow staff in England more time in their own classrooms.

Cover was "not an effective use of teachers' time", he added.

The updated rules follow an earlier agreement aimed at freeing teachers from administrative tasks like photocopying and reading the register.

'Languishing'

Speaking at a conference in London to promote workforce reform, Mr Miliband said: "These reforms are important and the prize is huge: teachers freed to teach, a respected profession and children benefiting from more personalised attention."

Steve Sinnott, general secretary of the National Union of Teachers, welcomed the limit but warned that proper cover was still needed.

He said: "A limitation on the number of hours teachers are required to cover can only be welcome.

"But, particularly in primary schools, the means of providing cover is to spread children round other classes, which is damaging to their education."

He said a NUT survey showed supply teachers were finding it "increasingly difficult" to get work.

"This wealth of experience should not be left languishing," he added.

Chris Keates, acting general secretary of the NASUWT teaching union, said: "At long last it has been recognised that cover is not an effective use of the time of teachers at a school.