Schools should go into the red to reduce teacher workloads and avoid being sued by their own staff, according to an Oxfordshire headteacher.

Clive Hallett, Oxfordshire secretary of the National Association of Head Teachers (NAHT) and head of Wheatley Park Primary School, said he was furious at Education Secretary Ruth Kelly's call for teachers to take legal action against heads who fail to free up teacher time.

Ms Kelly told the Association of Teachers and Lecturers that extra money being handed to schools was sufficient to free up 10 per cent of teachers' time for planning, preparation and assessment (PPA).

Mr Hallett said heads backed the idea but many, particularly those in small primary schools, would have to go into debt because they lacked the Government funding.

He said: "To suggest that teachers should sue their headteachers is nonsense. I'm incandescent about this. Everybody in education thinks that the implementation of PPA is a great idea.

"Teachers have been under huge stress since the introduction of the National Curriculum and league tables and they need this extra time. But it is not being funded and for the Government to say it is giving enough is a blatant lie. I would recommend heads implement it and put their budgets into deficit."

John Fisher, headteacher of the 430-pupil Rush Common Primary School in Abingdon, has been giving teachers the extra time for the past year as part of pilot scheme.

He said: "The funding is derisory and it has caused enormous problems for headteachers. But we have worked extremely hard to implement PPA time for the staff and it has gone remarkably well."

His view was backed up by Rush Common teacher Claire Mellor. For the past year, she has been given extra time out of the classroom while her teaching assistant takes over for half-an-hour periods.

Teaching assistants are not qualified teachers, but can perform tasks such as taking the register and getting pupils to recite times tables.

Mrs Mellor said: "We still come in at the same time every morning but having the extra time in the day makes it a lot more convenient.

"We used to work until at least 5pm and sometimes later than 7pm, but now we usually work until at least 4.30pm. We have an incredible team of teaching assistants and I'm confident leaving them in charge of the class. I don't know about teachers suing their headteachers if they don't give them extra time. It wouldn't occur to me."

Kidlington's Gosford Hill School headteacher David Jones said: "I'm shocked that our Secretary of State seems to be inflaming the situation with divisive comments when we should be working together.

"Like most secondary schools, we are already implementing these measures. It costs about £96,000 a year which means less money for other resources but we've made real progress in the last four years."

County council executive member for schools Tony Crabbe said: "Schools are not allowed to go into deficits without our approval and we would be very unlikely to agree to it for this reason."