The head of one of England's biggest exam boards has expressed "profound regret" after leaks hit two of its A-level exams in the past few days.
Rod Bristow said Edexcel's maths and economics exams had been overshadowed by concerns that content had been available beforehand.
Some of an economics exam appeared on social media and a maths paper was apparently for sale online.
Police arrested and released two men pending further inquiries in the case.
The alleged leaks prompted angry responses from teachers' and head teachers' leaders, who said this year's exams had been "plagued" with problems.
Pearson, which owns Edexcel, said the sale of maths exam papers had been confirmed on Friday.
But Pearson chief executive Mr Bristow said this was "too late to replace papers to be sat that day and the following Monday".
"We notified the exams regulator Ofqual and took the evidence we had gathered to the police, who immediately began their investigations.
"The police rapidly made arrests, but their detailed investigation takes time, and we had to make a decision about how to approach the exams that students still had to sit," said Mr Bristow.
"In any situation like this we have well-established contingency plans that adhere to one over-riding principle - fairness for all students.
"We have to ensure that the exams are secure, and that we will be able to mark papers to ensure fair results.
"If we had had any reason to believe that there had been a widespread breach of the maths papers, we would have postponed this week's sittings without hesitation."
As the police had suggested the problem was localised, the board decided to issue replacement exam questions for the papers to be sat in the small number of schools and colleges thought to be affected.
'Loss of confidence'
Staff visited those centres on Monday morning to help teachers issue the replacement questions, said Mr Bristow.
The issue of the economics A-level paper was quite different in nature.
The board was alerted after some content from the exam appeared on social media about 20 minutes before it was due to be taken.
Mr Bristow said: "This contained no precise details about specific questions and we do not believe any student has been disadvantaged, although we will monitor trends very closely."
Mary Bousted, general secretary of the Association of Teachers and Lecturers, said: "The reputation of exam boards relies on the security and effective administration of exam papers.
"But this summer's exams have been plagued with questions leaking, unanswerable questions and essential information missing from exam papers. Students, parents and teachers are likely to be distressed by these failings and lose confidence in the exam system."
Geoff Barton, general secretary of the Association of School and College Leaders, added: "We expect more of the examination boards, whose reputation rests in their ability to design and check exam papers, and then to guarantee their security.
"Schools and colleges pay thousands of pounds each year for this service."