By Gary Eason BBC News Online education editor

GCSE exam papers that half a million students are due to begin sitting next week are having to be reprinted, following the theft of a Parcelforce van.

Part of the van's load was English and English literature papers going to a school from the biggest exam board, AQA - which were later found to be in public circulation.

AQA - the Assessment and Qualifications Alliance - told BBC News Online it took the view that the exams had been compromised.

Papers to be taken by half a million students on 21 May and in early June - 75% of the national GCSE entry - were being changed and sent out again.

The estimated cost to the exam board is £250,000.

"As far as I'm aware this is the first time that anything of this magnitude has happened," said AQA spokesman George Turnbull.

Copies of these papers have been distributed in the Midlands and, possibly, further afield AQA letter to schools Exam season off to wobbly start

The revelation comes a day after the exams regulator, the Qualifications and Curriculum Authority (QCA), launched a campaign to reassure A-level students about this year's exam process, following the re-grading fiasco in 2002.

The authority said it had been kept informed of developments by AQA and was satisfied with its actions.

It has confirmed that papers and marking schemes for the Key Stage 3 national curriculum tests - taken by 14 year olds throughout England last week - were also on the van.

But a spokeswoman said that when the van was recovered, that package was intact - so no breach of security had taken place.

Replacements

West Midlands Police said they were investigating the reported theft of a Ford Transit van "apparently on hire to Parcelforce" which was taken at about 1pm on Thursday 1 May while parked outside the post office in High Street, Quarry Bank, Brierley Hill.

Parcelforce said it was talking with the exam board about the issue of compensation.

Mr Turnbull said AQA was alerted by Parcelforce on 6 May.

The board has written to the 3,000 examination centres involved, many of which had already received the exam papers.

They have been asked to fax back an acknowledgement that those papers are being scrapped.

The schools and colleges have been told they should get the replacements by 20 May, the day before the first exam.

AQA's letter to centres, dated 9 May, says: "Investigations involving the police, Parcelforce and AQA have now clearly established that copies of these papers have been distributed in the Midlands and, possibly, further afield."

As the biggest board, AQA has the lion's share of the GCSE exams - primarily in England, though some centres in the rest of the UK use its papers.

English is a compulsory subject at GCSE level, so "a massive number" of students were affected, Mr Turnbull said - half a million, taking 1.5 million exams.

"It's not an easy matter. It's quite a logistical effort on the part of the examination board.

"To make sure they are out in time for the 21st has been a considerable effort.

"But there's no room for compromise."

He added: "The important thing is that students are not affected."