Michael Gove's plans to cut school holidays and extend school days suffered a "huge blow" today after a body appointed to assess the proposals rejected them.
The education secretary had called for the removal of regulations limiting teacher hours, saying they were a relic of an era when school children were expected to also work in the fields.
However, the School Teachers Review Body (STRB) which advises the government on teacher pay and conditions, said the education secretary's proposals were unnecessary as teachers already work well beyond their set hours.
"We endorse the current provisions of 195 working days and 1265 hours," the chair of STRB Dame Patricia Hodgson said.
"We note that teachers currently work additional hours beyond directed classroom sessions and there is already flexibility for heads to deploy teachers according to the needs of their pupils.
The report highlighted concerns that attempts to extend school hours could kill off the culture of after-school clubs.
"Teachers who currently run after-school activities on a voluntary basis might not do so if working hours were extended," it found.
"This would mean that in future, schools would have to pay for work that is currently undertaken by teachers on a goodwill basis without extra payment and a likely consequence would be that many schools would have to stop providing these activities due to lack of funding."
They also noted that longer school hours and terms would require teachers to take more time out from lessons in order to prepare and plan.
Unions today welcomed the report's findings.
"The STRB has delivered Michael Gove a huge blow by rebuffing his recommendations for further attacks on teachers’ conditions and pay," general secretary of the National Union of Teachers Christine Blower said.
"Michael Gove sought to persuade the STRB that the teachers’ contract undermined professionalism and that the provisions were over-prescriptive. He failed on all counts."
The Association of Teachers and Lecturers also welcomed the rejection of longer hours for teachers.
"We are relieved the STRB has sensibly resisted pressure from Michael Gove to increase teachers' formal working hours and has also decided to protect the existing limits on what teachers are expected to do in their own time," general secretary Mary Bousted said.
The STRB agreed with proposals from Gove to remove regulations restricting teachers from conducting tasks such as bulk photocopying and filing.
They also agreed that schools should be given greater flexibility to give headteachers large salaries.
The Department for Education today accepted the conclusions of the report.
"Subject to a public consultation, we intend to accept all the key recommendations," a spokesperson said.