Headteachers last night demanded an independent inquiry into the growing A-level scandal - despite the exam boards being ordered to re-mark disputed papers.

Education Secretary Estelle Morris also stepped into the row. She said those papers suspected of being deliberately manipulated - to counter suggestions that A-levels had become too easy - should be re-marked.

Heads accused the exams watchdog, the Qualifications and Curriculum Authority (QCA) of pressurising the boards into marking down A-level entries, and said an independent inquiry should not be carried out by them.

Two exam boards, OCR and AQA, initially faced the allegations, but it has emerged that the third board, Edexcel, could also face similar claims.

David Dunn, head of independent Yarm School, Teesside, where several high-flying psychology students failed their course work, said the threat of legal action was as strong as ever.

"We are one of 30 independent schools who have strong evidence of injustice being carried out," he said.

"The HMC (Headmasters' and Headmistresses Conference) is going to take this all the way to an independent judicial review and some parents will be seeking damages out of this."

He said 14 papers had already returned re-marked with the original grades supported and that re-marking would solve nothing.

Headteachers now want all A-level entries re-checked according to the original grade boundaries and all results re-issued.

If the head's claims are true, it will become the biggest scandal to engulf the exam system in living memory.

John Heslop, regional officer of the National Association of Headteachers, said: "The examination boards should get the papers, have a marking system, mark them and tot up the marks. That should be it.

"They shouldn't be any tinkering with that to accommodate political pressures. If I was a parent, I would be absolutely furious."

The exam board climb-down came after Ms Morris met Dr Ken Boston, chief executive of th QCA.

The HMC's secretary, Geoff Lucas, will meet Ms Morris on Friday to discuss their grievances.

The boards have denied manipulation, saying they followed the QCA's code of practice and ensured standards remained comparable to 2001.