A further maths A-level paper due to be sat by about 7,000 candidates on Thursday has been replaced following the leak of an earlier exam last week.
Two questions from the Edexcel maths A-level paper were shared on social ahead of it being sat on Friday.
Pearson, the exam board's parent company, says it is replacing the latest paper and an unnamed centre is being investigated for the leak. It described the move as "precautionary steps" to protect students.
Pearson said their investigation had revealed a package containing the further maths paper had been opened by an individual at the centre concerned.
According to the company, there is no evidence to suggest the withdrawn test or any of its questions have been leaked but it is taking "precautionary steps" to safeguard the exam for the students.
Sharon Hague, senior vice-president, schools for Pearson, said: "We have reached out to all of our centres directly to inform them of this decision. We will continue to support and communicate with them through this unusual yet necessary step that is vital for the safeguarding of confidence in the examination system and to ensure fairness for all learners. Our message to students is not to worry about this and focus on your revision as you normally would."
Arrangements are being made to deliver the new further maths paper to all centres shortly before Thursday's exam - with the exception of the one being investigated. For this centre, separate arrangements are being made to ensure its students can complete their exams.
In a video message to students, teachers and parents, Ms Hague said it was necessary for everyone involved in the exam system to work together.
"We are reliant on the collaboration and trust of everyone involved in the exam system - and when someone commits malpractice, they let everyone down," she said.
She said the "serious security breach" last Thursday had been referred to the police, who had been asked to investigate it as a criminal matter.
Ms Hague added there were various ways to ensure fair outcomes from last week's A-level maths exam, including the option to exclude the two leaked questions from the final calculation.
Earlier this year, Pearson said it would be trialling a scheme where microchips were placed in exam packs to track the date, time and location of the bundles.
