Students who took an AS-level maths paper - which included a supposedly impossible question - answered that one better than any other in the exam, the board responsible claimed last night.

Edexcel was pilloried in January for a series of blunders which came to light after teachers complained about the question, and Education Secretary Estelle Morris sent in a troubleshooter to bring the board back on track.

But Trevor Easingwood, Edexcel's chairman of maths examiners, said almost 50 per cent of candidates scored 13 out of a maximum possible 14 marks, even though the question contained a "proofing error".

However, the North-East headteacher who was one of the first to highlight the mistake accused the board of the "worst case of spin".

Edexcel decided to give all candidates the maximum three marks for the part of the question it had already admitted was almost impossible to answer, said David Dunn, headmaster of Yarm School, near Stockton.

Last night, Mr Easingwood said: "The 'unanswerable' AS maths question was very answerable - in fact, it was answered better than any other question on the paper by a majority of students."

But Mr Dunn declared: "This is the worst case of spin. Edexcel told markers to give everybody the marks anyway and next they are saying that everybody did best on this question."

Edexcel acknowledged a "small minority" began answering question six all over again after they spotted the error, but insisted there was "no evidence to suggest that candidates ran out of time".

Mr Dunn responded: "I have got a number of pupils here who were thrown by the disruption."

One girl who had been heading for an A ran out of time "because of Edexcel's incompetence", he added.