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Papers from all three of the main exam boards may have been stolen, and more of Edexcel's than previously reported.

In all, the authorities are watching for signs of cheating in at least 20 exam units, some of them GCSEs.

They were in the secure centre of Harrow Park Tutorial College, a private college, which discovered the theft on Friday 11 June.

'Victim'

A spokeswoman said on Monday: "There was an incident at the college, and it was reported immediately to the police.

PAPERS INVOLVED The exam papers, A-levels and others, which the QCA says "potentially could be considered as having been stolen": AQA Accounting (ACC7) Business Studies (BUS6) AS-level Economics (ECN1, ECN2 and ECN3) Edexcel Biology and Human Biology Unit 4 A,B, C (6104/03) Human Biology Unit 5H (6105/01) Biology Unit 6 Synoptic (6106/03) Chemistry Unit 4 (6244/01) Chemistry Unit 4 Nuffield (6254/01) Chemistry Unit 6 (6246/01) Chemistry Unit 6 Nuffield (6256/01) Pure Maths P2 (6672/01) Statistics S2 (6684/01) Decision Maths D1 (6689) OCR GCSE: Science Double Award A (F) (1983/03) Science Double Award Chemistry (H) (2413/02) English: Different Cultures, Analysis and Argument (2432/01) AS-level: Wave Properties/Experimental Skills 1 (2823/01) Physics in Action (2860/01) Understanding Processes (2861/01) The Market System (2881/01) Market Failure and Government (2882/01) National/International Economy (2883/01)

"The staff and students are very angry that they should have to sit examinations knowing that somebody has cheated."

No staff or students at the college had been interviewed by police over the theft, she said - which has surprised the exam boards.

"We're nothing to do with the sale of exam papers. We are just a victim of crime," the spokeswoman said.

"I do hope that they can catch whoever is responsible."

The Edexcel board said it went in to investigate the theft on 11 June because its offices, in London, were the nearest to the college.

A spokeswoman said that it carried out a full audit over the weekend. By Monday it had confirmed that some of its papers were also missing, and added that information to the police inquiry.

It removed all the remaining exam papers from every board from the college's store - they have been returned as needed on various days since so students could sit their exams.

Some of the stolen papers have turned up, allegedly being sold between students.

Last week two newspapers obtained some chemistry, maths and biology papers, and questions from a maths paper were posted on the internet.

All of those were Edexcel papers. The board has changed part of a chemistry exam due to be taken this Tuesday.

Following an inquiry from BBC News Online, it said on Tuesday that, due to "a simple administrative error", it had omitted the Chemistry Unit 6 papers from the list of those stolen from the college.

Cheat watch

AQA's spokeswoman said it had issued replacements for two of its papers - but would not confirm which had been taken.

But she added: "None of ours appear to have come to light. We haven't had any reports of any circulating."

The exams watchdog, the Qualifications and Curriculum Authority (QCA), has issued a list of the papers which it says were potentially involved.

A spokesman said examiners were on the alert for signs of cheating, but the authority stood by the general approach of not changing exams even though papers had been stolen.

"To change an exam you need clear evidence of a security breach," he said.

And it took time to recall and replace papers across the country - which was very disruptive for schools and colleges.

"There is a lot of experience of monitoring students' performance when there's a suggestion of a security breach and the sorts of things that happen, particularly at A-level - pupils' past performance can be taken into account."

Procedures

He added that teachers were among the main guardians of the integrity of the exam system.

"Their input is important in a case like this - they don't want their honest pupils to be disadvantaged."

But a spokesman for the OCR exam board - while not able to comment on the college involved in this incident - said that in general some schools could do more to improve their security procedures.

"Some really do need to tighten them up," he said.

"It is an issue."