And the Qualifications and Curriculum Authority (QCA) is promising that by Friday it will have completed an inquiry into allegations that grades are being manipulated.

There have been claims that marks have been lowered to prevent accusations that getting a top grade at A-level is too easy.

A-level investigation

Exam watchdog reviewing marking in OCR exams in: English physics chemistry history geography design and technology French German psychology

"We can state categorically that there has been no instruction by QCA to depress grades artificially and disadvantage students," said the chief executive, Ken Boston.

The exam watchdog says that it will check results for A-levels set by the OCR exam board in English, physics, chemistry, history, geography, design and technology, French, German and psychology.

"We are looking at them closely to see if there is any pattern of deflation in coursework results," says Mr Boston.

Pressure

And any politically-motivated involvement in the dispute over A-level grades has been dismissed by the Prime Minister's official spokesperson as "utter rubbish".

But pressure continues to grow for an independent investigation into claims from schools that A-level grades have been unfairly downgraded.

Damian Green says dispute threatens confidence in system

The Conservatives' education spokesperson Damian Green says that the claims over result fixing "strike at the heart of confidence in the exam system".

And he voiced suspicions that results were being "fiddled in retrospect" to suit a political agenda, which wanted to avoid the appearance that A-levels were becoming too easy.

Urgent

But the exam watchdog says that claims that there has been a deliberate message sent to markers to lower grades is "implausible" and the "least likely" explanation for the disputed papers.

Not only do I believe there should be an urgent inquiry ... but I think claims for compensation arising from a failure by students to get into the university of their choice should be taken seriously

David Hart, National Association of Headteachers

Head teachers remain convinced that the exam boards need to be investigated over the disputed marks.

And representatives from the Headmasters' and Headmistresses' Conference of leading independent schools, will be meeting OCR on Thursday.

Head teachers are threatening legal action if they are not satisfied by response from the exam board.

The disputed results, many of which are from independent schools, typically involve pupils who have scored very highly in written exams - but then have received very low marks for coursework.

This has lowered their overall grade - and has put university places at risk.

Zainab Khan, a former pupil at Colston's Girls' School in Bristol, is contesting coursework marks in Spanish and history, which threaten to prevent her from taking up her place studying law at university.

Despite high scores in other parts of the exams, including a 100% for one section, she has scored E grades for coursework in both.

The Spanish coursework was seen by the school before submission and was assessed by teachers to be excellent - and an appeal to the chief examiner is in process.

"It has been really stressful. You read about this kind of thing, and think that it must be the student who hasn't performed well.

"And if it was an exam, I'd be prepared to accept that I had done badly, but coursework, which has been seen by teachers, is different," she said.

'Unacceptable'

David Hart, general secretary of the National Association of Head Teachers, also criticised exam boards for attempting to blame teachers for the results because of "over optimistic" assessment of students' work.

Mr Hart said: "Not only do I believe there should be an urgent inquiry into the complaints about the marking of A-level subjects but I think claims for compensation arising from a failure by students to get into the university of their choice should be taken seriously."

The OCR exam board confirmed that the number of requests for inquiries into grades for its exams had jumped from 1,600 last year to 4,000.

The board's chief executive, Dr Ron McLone, is to meet leading private school heads on Thursday to discuss their concerns.

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