Sixth-form students may be forced to take three A-level exams in a day if a system of post-qualifications applications is introduced, an exam body chief has warned.

Simon Lebus, group chief executive of Cambridge Assessment, said plans to shorten the school year and compress the exam timetable to allow enough time for students to make an application with their results in hand could put undue stress on candidates.

Under proposals published by the Universities and Colleges Admissions Service last month, students would sit their exams 15 days earlier, starting in mid-May, and receive results in mid-July before applying.

But Mr Lebus, who is also chairman of the OCR Exam Board, said squeezing exams into a five-week period, instead of seven weeks, would put extra pressure on students.

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"You move to a situation where students who currently take no more than two exams a day are taking three a day," he said. "Timetable conflicts are very difficult to [avoid], so compressing the timetable will make it harder."

He also warned that a 13 per cent cut in the timetable period would have a "wash-back effect", with revision classes having to be scheduled in the spring term, which would reduce teaching time.

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The three-week "make your mind up window" proposed by Ucas could also discriminate against risk-averse students from poorer backgrounds, Mr Lebus added.

"My own sense is that it might act against those with less social capital," he told the University Access and Admissions conference in London last week.

Sion Humphreys, policy adviser for the National Association of Head Teachers, believed the proposed system would place too much "pain" on schools, rather than universities. "Most of the resources and costs seem to be laid at the feet of schools and colleges and relatively little at the door of higher education," he said. "Moving the university term back a week might be an idea."

Arguments that PQA would encourage more teenagers from poorer backgrounds to apply to university were also criticised.

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Champions of the approach have claimed that poorer students tend to opt for less prestigious universities because they do not feel that they will achieve their predicted grades. When they achieve top marks, it is too late to reapply.

Graeme Atherton, head of AccessHE, a London-based outreach programme, said: "I don't see it making a big difference in terms of widening participation. If you have limited resources, we would not focus on this area."

Usman Ali, vice-president (higher education) of the National Union of Students, agreed. However, he welcomed the changes, saying a system based on predicted grades was flawed: "It's not about making the system fairer, it's about making it more effective."

Matthew Andrews, academic registrar at Oxford Brookes University and chair of admissions for the Academic Registrars Council, added that selection on actual grades was preferable. "If someone proposed we gave out medals for the high jump at London 2012 based on predicted heights of jumping, we would think it was ridiculous."

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Mary Curnock Cook, chief executive of Ucas, said the 50,000 candidates who chose to reapply this year after receiving their grades highlighted a fault in the system.

jack.grove@tsleducation.com.