Mr Woodhead has said that an affair with Amanda Johnston, more than 20 years ago, began after he had left his teaching post and Ms Johnston had ceased to be a pupil at Gordano School near Bristol.

The newspaper's report contradicts Mr Woodhead's earlier claims

But Mr Woodhead's ex-wife told a newspaper that the affair began while he was still a teacher and Ms Johnston was still a pupil at the school. Mr Woodhead denies the claim.

David Hart, General Secretary of the National Association of Head Teachers, says that there must be an "urgent inquiry" into the events to establish the truth.

"He went on the record denying he had an affair with a pupil when he was a teacher. Now his ex-wife has issued a statement to the contrary.

"If he lied, clearly his job must be at risk. It cannot be anything else. It is now in the interests of all concerned, not least in the interests of Ofsted itself, for this to be resolved."

Another union leader, Peter Smith, general secretary of the Association of Teachers and Lecturers, said: "I think that if his ex-wife is saying he has lied in public, simply for him to say 'no comment' is not an adequate response."

Cathy Woodhead, who was divorced from Mr Woodhead in 1977, was reported in the Mail on Sunday as saying that she decided to speak out after her ex-husband told students in January that relationships between teachers and their pupils could be "educative".

She told the paper: "At first, I couldn't believe what I was hearing. Then I was simply incensed.

"Yes, I thought, while you were away being educative and experiential with someone else, I was looking after our baby daughter.

"He angered me further by repeating his lies about when the affair started, thus effectively denying the reality of what was for me a profoundly painful episode."

Mr Woodhead subsequently apologised for his remarks, and said there was an "enormous difference between having a relationship after a teacher has stopped teaching a pupil and having an affair with that pupil while a teaching relationship is still going on".

'Not drawn'

Mrs Woodhead also reportedly said her ex-husband suggested she and his girlfriend should both live with him under the same roof.

Their daughter Tammy was aged 15 months at the time and Mrs Woodhead told the paper: "Chris said I should consider myself lucky I would have both him and Amanda to look after Tammy."

Mr Woodhead refused to comment on the allegation that he had publicly lied about the affair.

He said: "I have absolutely no comment to make in addition to what Amanda Johnston and I have already said.

"She (my ex wife) can say whatever she likes. I am not going to get drawn into this."

Ms Johnston's brother, Michael, also refused to confirm or deny Mrs Woodhead's claims.

Mr Johnston, 47, from Portishead, Somerset, said: "Chris Woodhead was virtually a brother-in-law for nine years and I have nothing against the guy."

Response attacked

Mr Woodhead has insisted previously the affair started when he and his former pupil met again in Oxford after they had both left the school.

Amanda Johnston also said last month that she and Mr Woodhead had had the relationship some years after he taught at the school.

'Unfortunate' remark

The government backed Mr Woodhead when he made his controversial remarks about pupil-teacher relationships in January.

Education Secretary David Blunkett said it was an "unfortunate" remark but did not prevent him from doing his job properly.

The Department for Education said it had no comment on the Mail on Sunday report. Downing Street also declined to comment.