Hundreds of A-level students had to sit a replacement exam this week after some teenagers tried to cheat by getting hold of the questions in advance.
The AQA exam board launched an investigation and has warned the would-be cheats that they face disqualification from these and future exams.
About 200 business studies A-level candidates were affected by the incident at a small number of colleges and schools in London.
The board discovered last week that some students had seen the exam.
"We took immediate action and arranged for candidates in those centres to sit replacement papers," an AQA spokeswoman said.
"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.
"Any candidates who are found to have cheated risk disqualification from this - and future - exam series.
"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."
She said the board was looking at security procedures at several centres and interviewing a number of candidates.