A-level students who logged on to a website where stolen exam questions were posted could lose their university places if they are found to have cheated.
The exam board Edexcel said they would be disqualified and their grades withdrawn after it discovered papers in maths and chemistry had been stolen.
Two questions from a pure maths paper sat on Monday were posted on the net the day before, the board said. Police are working with the board and internet service provider and have identified the source of the net questions.
An Edexcel spokeswoman said those who logged on to the page had also been identified: "A number of people logged on to the site. It was not a vast number. But we will be subjecting their papers to extra scrutiny."
The questions were removed from the site on Monday morning when the discovery was made.
The spokeswoman said: "We want to reassure those who have genuinely worked hard that they will be all right but the cheats will be found and disqualified."
In another incident the Daily Mirror handed Edexcel pages from four A-level maths and chemistry exam papers. A student was believed to have got hold of them. Next Tuesday's chemistry exam will have to be substantially changed, the board said. More than 8,000 students in the UK are sitting the examinations affected.
At pound;6,674-a-term Epsom college 13 sixth-formers taking an English AS-level re-sit on June 7 opened their Shakespeare papers to find questions on As You Like It rather than the text they had studied, The Merchant of Venice.
English teachers at Epsom put together a paper on The Merchant of Venice which pupils sat later after OCR said it could be used to support an application for special consideration.
Jane Britton, marketing manager of the college said: "Pupils were understandably concerned about the situation but were confident that their teachers would do all they could to make the best of it."