Questions had to be replaced on two A-Level exam papers amid allegations of malpractice.
Pearson examinations body confirmed it had changed questions on Monday's statistics and further pure maths A Level papers in schools where it was claimed "some students had information they should not have had".
The organisation said it takes the allegations "extremely seriously".
Of Monday's changes a Pearson spokesman said: "We made the decision to replace some questions in two of this afternoon's examinations in the small number of schools and colleges where we'd heard allegations that some students had information they should not have had.
"Replacing these questions helps us to ensure the integrity of the examination.
"The vast majority of students work hard at their studies and want to take exams safe in the knowledge that they are working to a level playing field.
"We are continuing to investigate the allegations of exam malpractice, which we are taking extremely seriously.
"We would like to thank those individuals who have already come forward with information and they can rest assured that we will take every action necessary to ensure the integrity of these examinations and that all students will be marked fairly.
"Any school college or student with concerns can contact us at firstname.lastname@example.org."
It follows allegations of "malpractice or wrongdoing" made concerning a maths paper, run by Edexcel which is owned by Pearson, taken on Friday.