The Qualifications and Curriculum Authority (QCA) has revealed that 69,226 candidates were given more time in 2007, compared with 35,319 in 2005. Last summer’s statistic is a 22 per cent rise on 2006.

Teachers can allow pupils up to 25 per cent longer in the exam hall without seeking approval from the authority. Extra time is allowed for conditions such as dyslexia and dyspraxia.

But approval is required if more time is needed. Last year, 2,489 requests for more than 25 per cent extra time were approved - a 4 per cent rise on 2006.

Exam regulations suggest it is not easy for pupils to gain extra time. Parents have to provide schools with a certificate provided by a psychologist or a specialist teacher confirming that the pupil’s need is genuine.

It is unclear what is driving the increase, but there was speculation this week that the pressure on schools to raise results may be partly responsible.

Professor Alan Smithers of Buckingham University said: “One explanation is the exam results are very important to schools, and they are taking advantage of every flexibility available to them.”

Isabel Nisbet, the QCA’s director of regulation and standards, said the rise could be attributed to schools being more aware of the flexibility, but said she needed to investigate to make sure the system was not being abused.

Shirley Cramer, chief executive of the charity Dyslexia Action, said the Joint Council for Qualifications could consider tightening the rules.

Steven Poole, head of Hinchley Wood Secondary in Esher, Surrey, said parents were becoming more aware that it was possible for pupils to be given extra time in the exam hall if they were entitled to special consideration.

Overall, GCSE and A-level students made nearly half a million requests for special consideration in last summer’s exams.

The 470,580 requests include demands for modified question papers, readers to dictate questions, and scribes; the figure represents a rise of 7 per cent this year. One in three was approved by the boards.

There was also a drop in the number of exam cheats caught last year - 4,227 pupils penalised, a fall of 11 per cent on 2006.

The most common offences were bringing unauthorised materials into the exam hall - often mobile phones or crib sheets - copying and plagiarism.

In national tests, the number of alleged cases of “maladministration” - pupils or teachers cheating or other breaches of exam rules - fell by 8 per cent last year to 532.

Some 102 sults. Four schools had all their results annulled.

Exam boards were reported as providing satisfactory service overall to teachers, achieving nearly all the performance targets the QCA had set for them.