In one of the sections, candidates had been expecting to answer questions about crime and punishment in Anglo-Saxon times, but were instead asked about the role of the church in medieval justice.

This is very bad and I have to say if this had been in the earlier sequence of GCSEs it could have had a profound effect on their confidence in the system

David Johnson, head teacher

The Oxford and Cambridge and RSA board (OCR) said 2,554 out of a total of 28,617 pupils taking the course had been affected.

The board said the marking scheme would be adjusted to ensure that candidates were not disadvantaged.

It would also be inviting a representative from each school involved to attend a meeting on how grades would be awarded.

Apology

"OCR apologises to candidates for the distress and confusion caused when they were faced with questions on a topic area they were not expecting," a spokesman for OCR said.

"We can assure all candidates that they will not be disadvantaged by this error," the spokesman said.

"This was primarily a skills-based paper that tests candidates' use of historical source material.

"Consequently, candidates were still able to attempt the questions using the source material provided.

"Candidates' answers will be marked on their handling and use of the source material," he said.

A spokesman from the Qualifications and Curriculum Authority said: "QCA is aware of an error in an OCR GCSE history examination paper and is immediately beginning a formal investigation with OCR."

"The first priority is to make sure that things are swiftly put right. No student will be disadvantaged by this error," he said.

'Shocked'

The head teacher of Fair Oak High School in Rugeley, Staffordshire, David Johnson, said 65 of his pupils were affected by the error.

"These candidates must be looked after at all costs - they have been left shocked and surprised," said Mr Johnson.

"They will not have done their best work and in fact they haven't studied the period in question. They couldn't do their best work.

"This is very bad and I have to say if this had been in the earlier sequence of GCSEs it could have had a profound effect on their confidence in the system."