Sir William Stubbs, chairman of the Qualifications and Curriculum Authority (QCA), said any attempt at such interference would probably be a resigning matter as far as his board was concerned.

He was commenting on the claims that A-level grades were deliberately lowered this summer.

The QCA's new chief executive, Ken Boston, is conducting an investigation into the OCR exam board and has promised to get the results to the Education Secretary, Estelle Morris, by Friday.

'No pressure'

There have been claims that marks were lowered - especially in coursework - to prevent accusations that getting a top grade at A-level had become too easy.

As it was, the pass rate jumped by 4.5 percentage points to 94.3% this year - the first year of the new-style A-levels .

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," Ken Boston said on Monday.

Sir William said on Tuesday morning: "To be absolutely clear, there has been no pressure, of which I am aware, on the QCA or on boards from the secretary of state, from ministers or from senior civil servants about grades.

"That is a matter on which they are quite meticulous and they keep themselves away from.

Resignations

"If there was any political interference with the individual results of boards or students then the QCA board would take that most seriously and I expect resignations would follow.

"But I can say that, confident that there has not been that kind of interference," he added.

The QCA says it is checking results for A-levels set by the OCR 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."

Pressure

Any politically-motivated involvement in the dispute over A-level grades was also 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, has said 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

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 Head Teachers

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.

Law place at stake

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 at risk university places.

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.

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