Independent school heads said they were forced to challenge grades because of sloppy marking. They blame an insufficient number of examiners to deal with the biggest summer of exams ever.

University places for some sixth-formers had been placed in jeopardy because of late grade changes.

Graham Able, head of Dulwich College in south-east London, said: “We have seen many more inquiries and I think we will see many more changes in grades than we have seen before.

“To my knowledge there are cases in all three exam boards where the 30-day fast-track marking procedure has not been adhered to.”

Troubled exam board Edexcel was unable to provide figures on the number of complaints or how many were dealt with on time. The future of chief executive Tina Townsend is in doubt after discontent over the board’s preformance in this summer’s exams. Oxford and Cambridge Board and Royal Society of Arts, and the Assessment and Qualifications Alliance have denied they had been slow dealing with urgent appeals.

OCR said all fast-track complaints from students waiting for university places had been completed within the 30-day limit. AQA said more than 95 per cent of urgent inquiries were dealt with on time. Schools were told to submit urgent appeals by August 25.

The boards also dispute accusations of poor marking. A-level re-mark requests to AQA were down nearly a third this year, although added to AS-levels, the total of re-marks was up 18 per cent. OCR said it had received 11,931 inquiries this year, compared to 11,832 last year.