Education Secretary Charles Clarke yesterday made a last-minute concession to Labour rebels fighting the Government's plans to introduce variable university tuition fees.

He promised a legal bar on increases to the proposed £3,000 cap on fees for the life of the next Parliament.

As Cabinet colleague Margaret Beckett warned that Labour was "approaching the abyss" over fees, Mr Clarke hoped that this last-ditch concession would win over enough rebel MPs to secure victory for the Government in the knife-edge Commons vote on the Higher Education Bill tomorrow.

The vote is first of two dramatic events this week that threaten to destroy Tony Blair's premiership. On Wednesday, Lord Hutton will publish his report on the death of Iraq weapons expert Dr David Kelly.

Critics of fees have warned that elite universities will lobby for the £3,000 maximum annual charge to be raised to as much as £15,000, creating a two-tier system in which the best courses will be beyond the financial means of students from poor backgrounds.

But Mr Clarke said an amendment to the controversial Bill will explicitly state that the cap cannot be raised until after two general elections.

Coupled with the promise of an inquiry into the impact of top-up fees three years after their proposed introduction in 2006, this would mean the cap could not be increased in real terms until 2010 at the earliest, he said.

"I think that gives the kind of reassurance people were looking for who have been worried whether there is some secret plan - which there never has been, of course - to increase fees earlier than that," he told BBC1's Breakfast with Frost.

But he refused to budge on Labour rebels' key concern - the ability of universities to charge varying amounts for different courses.

Mr Clarke's latest initiative came as Chancellor Gordon Brown made his most high-profile intervention in support of the fees proposals.

Mr Brown urged Labour MPs to give full support to the Prime Minister in what he termed "a sensible, radical reform".

The question of whether Mr Blair should go into the next election as Prime Minister was "a matter for him", he told GMTV.

Mr Blair warned in an interview with The Observer that if the university funding reforms were rejected, Britain would be condemned to a second-class higher education system.

Both Mr Brown and Mr Clarke said they believed the Government would win tomorrow's vote.