Examiners shared "inappropriate" information about a GCSE paper due to be taken by pupils in January, the exams regulator warned today.
Ofqual said an information and communications technology (ICT) paper set by the WJEC exam board had been compromised, and will be withdrawn to protect the "integrity and security" of the qualification.
The finding was included in an urgent report ordered by ministers earlier this month in the wake of allegations that examiners had been secretly advising teachers on how to boost GCSE and A-level results.
In its report, published today, Ofqual said the ICT paper will be amended and sat at a later date.
The move will not impact on when students will be able to complete their GCSE course in the subject.
Three examiners, two from WJEC, the Welsh exam board, and one from Edexcel, were suspended as inquiries began earlier this month into claims that teachers were given unfair advice and told that one exam board set easier tests.
The allegations, made in the Daily Telegraph, centred on teachers being given detailed advice at exams seminars on forthcoming exam questions and how students could score higher marks.
Education Secretary Michael Gove ordered Ofqual to look into the claims and report back by Christmas.
The regulator's report found that, at a WJEC seminar on part of a GCSE ICT course, attendees were told which topics pupils would be assessed on.
"This is in clear breach of regulatory requirements, and it compromises the planned exam," the report said.
Around 450 pupils were due to sit the paper next month, and schools and colleges will be informed of its withdrawal.
The report also says that the Telegraph had highlighted concerns in particular about WJEC GCSE history.
No papers for this qualification are due to be taken in January, Ofqual said, and issues relating to this course and others will be investigated at a later date.
Exam boards have also been ordered to put in extra controls on seminars, and told to review their published guidance and seminar material on exams.
Ofqual chief executive Glenys Stacey said: "This update outlines our initial plans and findings and the actions taken to protect future exams.
"Where we have found evidence that a question paper has been compromised we have taken action. We are continuing our work to study the material provided by the Daily Telegraph. There is no evidence at this stage that other exams in January are similarly affected."
Ms Stacey later told BBC News that Ofqual had not found evidence that any other paper had been compromised, and that they are due to complete their review by the end of the week.
According to the Telegraph's investigation, teachers paid up to £230 a day for seminars hosted by examiners.
During some of these seminars they were allegedly given advice on the wording students should use to increase their marks, and which questions they were likely to face.
Undercover Telegraph reporters attended 13 seminars run by exam boards, the newspaper reported.
It alleged that at these seminars, teachers were "routinely" given information about upcoming questions, words or facts that students should use to gain marks, and areas of the syllabus that teachers should focus on.
In today's report, Ofqual said that, although there were instances where the evidence reviewed to date backs up some of the allegations, most of the material does not show unacceptable practice.
"Nonetheless, this raises questions about the role of such seminars which we will need to consider," it adds.
In the wake of the claims, WJEC suspended two of its history examiners, Paul Evans and Paul Barnes, both of whom were named in the Telegraph's report.
In one case, Mr Evans was alleged to have been recorded telling teachers that a compulsory question in a certain exam goes through a cycle.
He is said to have added: "We're cheating."
"We're telling you the cycle (of the compulsory question). Probably the regulator will tell us off," the newspaper reported.
Following a second story, a third examiner, Steph Warren of Edexcel, was suspended.
It came after the Telegraph released video footage of a conversation between an undercover reporter and the examiner who claimed the company's GCSE geography tests were not as difficult as those from other exam boards.
Ms Warren also described her disbelief that the test had been cleared by the exam regulator.
An Edexcel spokesman said today: "We are clear that awarding organisations can and must improve the way that training sessions are conducted and have made a public commitment to do that in very practical ways.
"Edexcel welcomes any review of the current examination and qualifications system that puts students at the top of the agenda.
"We intend to play an active part in any such review, which must consider the examinations system in the context of pressures placed upon it by wider education policy including league tables and planned reforms to the National Curriculum."