Hundreds of A-level students have had to sit a replacement exam after it was discovered papers had been leaked.
Up to 300,000 students are taking A-levels this year
Some schools and colleges in the London area were given alternative business studies questions by the AQA assessment board on Monday.
It has been alleged that some of the missing papers were put up for sale.
An AQA spokeswoman said the leak was an "extremely serious matter" but that the "vast majority" of candidates, who had worked hard, would be unaffected.
It is thought the security breach was at a school or college in London.
The AQA spokeswoman said: "We took immediate action and arranged for candidates in those centres to sit replacement papers on Monday: this action ensures that candidates could gain no advantage if they had seen the original paper beforehand.
"It equally ensures that the vast majority of candidates who had studied legitimately, with no intention of cheating, would not be disadvantaged."
An A-level student said: "People are worried that, if papers are leaked, the grade requirements might rise to compensate.
"They are angry that security has been breached. It's hard enough doing A-levels as it is, without all this."
The AQA said any candidate found to have cheated would be disqualified from this and all future exams.
The spokeswoman added: "This is an extremely serious matter and our action must ensure that we protect the interests of all our candidates and maintain the integrity of the examination system.
The board was looking at security procedures at several exam centres and interviewing "a number of candidates".
Up to 300,000 people are taking A-levels this year.
Were you due to sit the business studies paper? Were you affected by the exam paper leak? Did you have to take the replacement test?
I had to sit the business studies paper but I was not in London. Over here we had to take the papers as normal but I have heard of many leaks. I am pleased I did not have to take a replacement test as the exam was easy. Those that cheated ruined a nice straightforward exam for the people in London and I am upset with these people.
I was sitting the business studies paper on 5 June. I find it very unfair that people they are cheating here are students who are putting in time and dedication. A minority of students are cheating. I would just like to say that i would rather fail then cheat.
I did the exam this afternoon. Not sure if we were affected, though - no one told us anyhting.