The government has promised to launch a teacher recruitment drive after the new figures were revealed by The Sunday Times.

Teachers' unions have estimated as many as 500 primary and secondary schools will be without permanent head teachers when Britain's children go back to class this week. Most will have to rely on acting heads for several months.

Union leaders say education standards will not be raised without more teachers

A Department of Education (DfE) spokesman said they were unable to confirm the figures but said they did recognise there were teacher shortages, especially in maths and the sciences.

Last month plans were announced to reform the teachers' pay system to give better money and improved status to those who choose to stay in the classroom.

More money

Under the proposals successful teachers will be given an extra £2,000 a year and the best heads could earn up to £70,000.

Ministers also announced a £130m short-term package to tempt graduates into teaching in areas where there are chronic shortages.

New recruits are being offered a one-off payment of £5,000 if they agree to stay and teach in one of the problem areas, such as maths.

A DfE spokesman said: "We recognise there are difficulties but we have a strategy to deal with them and are determined to do so."

'Shortages affecting standards'

But the unions say the shortages could undermine the government's drive to raise standards in schools.

Nigel de Gruchy: "Supply teachers cannot fill the gaps properly"

General Secretary of the National Association of Schoolmasters and Women Teachers, Nigel de Gruchy, said: "More schools are having to rely heavily on supply teachers who are fine to cover sickness but are now being used to fill permanent gaps."

Vice President of the National Association of Head Teachers, Chris Thatcher, told The Sunday Times: "Some schools are being left without an experienced head for a significant time before a replacement can be found.