The UK’s biggest exam board has confirmed that 78 students have had their exam results withheld following a leak of an A-level maths paper this summer.

The security breach, which happened before tens of thousands of students sat an Edexcel maths paper on 14 June, prompted uproar among candidates concerned they would be at an unfair disadvantage.

With A-level exam results due to be published next Thursday, Edexcel’s parent company, Pearson, said that all of the maths papers had been reviewed in detail to ensure that candidates got a fair grade that reflected their work.

Exam security has become a problem in recent years, with smartphones and social media enabling cheats to disseminate leaked questions quickly, causing huge stress among students every summer. Such is the level of concern that a review of exam malpractice is under way and is due to report this summer.

At the time of the maths A-level breach, Pearson said just two questions, which were heavily blacked out, had been circulated on Twitter before the exam, which went ahead amid speculation that the breach had in fact been more widespread.

On Friday, the company confirmed that the entire paper had been shared by a number of students on a closed social media network before the exam was sat. “Based on the evidence available to us at that point we had to decide quickly whether or not to replace the paper or reschedule the exam,” said Sharon Hague, Pearson’s senior vice-president for schools.

“It was too late to replace papers and rescheduling would have caused significant additional stress and disruption for tens of thousands of students across the country. We knew that our expert assessment team would have the ability to perform detailed statistical analysis once marking was completed to ensure no one was disadvantaged.”

Pearson said the source of the breach was narrowed down to a single examination centre. Individuals were interviewed and their phones examined. The incident was reported to the police who seized equipment from two people who were subsequently arrested. “This police case is ongoing and we hope it will end in a criminal prosecution,” said Hague.

The 78 students at the heart of the investigation are having their results withheld while the board’s malpractice procedures are completed.

Pearson also addressed widespread complaints from students and teachers that the second Edexcel A-level maths paper sat this summer was too hard. Following a review, it admitted the first two questions were more challenging than might be typically expected of initial questions but insisted overall the paper was fair.

“We do appreciate that the experience of sitting this paper, for some students, was not what they had expected in terms of the perceived level of difficulty,” said Hague, who added: “We want to reassure you that independent experts have analysed the papers and confirmed paper 2 was a fair and valid paper, testing across the ability range and the course curriculum.”