Universities could struggle to meet strict benchmark targets on widening participation set by regulators because of a potential flaw in the admissions system, it has been claimed.

The warning comes after the government's announcement last week that universities wishing to charge more than ?6,000 in tuition fees must meet "stringent" new agreements set by the Office for Fair Access.

David Willetts, the universities minister, said the agreements would have to include commitments on outreach activities and financial support for poorer students and would be judged against "access benchmarks". He added that they would be backed up by a "tougher regime of sanctions", under which Offa could redirect a university's fee income towards access projects if benchmarks were missed.

However, a source concerned with the issue of widening participation told Times Higher Education that such strict targets may be hard to achieve for many institutions because of the current admissions process.

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The source, a professor at a research-intensive university who did not want to be named, said the ability of the universities of Oxford and Cambridge to interview candidates meant it was easier for them to control admissions.

As many students nominate the two prestigious institutions as their first choice, a relatively large proportion of applicants from richer backgrounds could miss out on places, he said.

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This would cause a knock-on effect where other "second choice" universities were flooded with high-quality candidates who missed out on Oxford and Cambridge, but who did not fulfil the access criteria. Although many universities could try to widen participation by lowering grade thresholds for certain applicants, this might become impossible at a time when they are trying to maintain reputation in a high-fee market, the professor added.

"What students are really paying for is the badge of reputation and that badge is provided by being selective. In that environment most universities will be wary of lowering offers," he said.

The issue could unfold in a similar way to a problem in the 1980s when a move to accept more women at prestigious institutions led to male students from England squeezing out Scottish applicants at universities such as Edinburgh.

Meanwhile, others have questioned whether the new widening participation criteria will have any "teeth", given the rush to bring them in ahead of universities setting fee rates for 2012 next spring.

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"It is very difficult at the moment to see anything that suggests the criteria are going to be significantly more stringent than the powers Offa has already enjoyed," said John Denham, the Labour shadow business secretary.

He added that it was also "unacceptable" that the Commons was being asked to vote on a rise in the fee cap without consideration of major structural changes to the sector that will be proposed in the government's White Paper.

"There are so many unknown questions which for some MPs are at the heart of this - for example whether the new widening participation criteria are really going to bite," he said.

simon.baker@tsleducation.com.