Critics who claim that universities are "dumbing down" have been told to "put up or shut up" by the chairman of a cross-party panel of MPs that is conducting a major new inquiry into standards.

Degree classifications, university admissions and student completion rates will all be investigated by the Innovation, Universities, Science and Skills (IUSS) Select Committee, along with the balance between teaching and research and the mechanisms of student support.

The investigation aims to confront head-on claims of slipping standards, and the sector has been invited to submit evidence in any of the areas under scrutiny.

Phil Willis, the committee chairman, said: "There are accusations that the move to a volume-led ÖйúAƬ system has led to dumbing down.

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"It is right and proper that we challenge those critics, and they have now to produce the evidence. This is a real challenge to those critics to actually come out of the woodwork, and I think universities can only benefit from that."

The launch of the inquiry into standards follows a recent Times ÖйúAƬ poll on the "dumbing down" debate, which found evidence that academics do believe standards are declining.

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Of the 500 readers who took part in the survey, nine out of ten participants said resourcing constraints were having an adverse effect on academic standards, and seven out of ten said the rise in the number of top degree classifications did not indicate an improvement in standards.

The IUSS investigation will conclude before the Government's review of tuition fees in 2009, and Mr Willis said it was important that any proposal to lift the fee cap could be justified with evidence that UK ÖйúAƬ remained "as good as anything else you can get in the world".

"A market has developed in ÖйúAƬ, students are now customers, and we want to ask what the value of the university experience is and whether the product those students get stands up to the tag of 'world class'," he said.

The inquiry comes three months after Peter Williams, chief executive of the Quality Assurance Agency, was called to give evidence to the committee amid concerns about the watchdog's ability to regulate university standards.

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Mr Willis said that Mr Williams had announced that he would "go away and talk to the universities about drawing up a plan to deal with some of those criticisms", but that he had not yet done so.

"That is a worry, but (Mr Williams) has written to assure me that, despite the very high-profile evidence session, the number of people who have come forward with hard evidence to support claims that the system is somehow corrupt have just not emerged."

Geoffrey Alderman, professor of politics and contemporary history at the University of Buckingham, who has been critical of "slipping standards", welcomed the review.

Professor Alderman said: "I hope that a large number of academics and support staff - especially those who have contacted me in recent months - will submit evidence. I am preparing mine."

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