Tony Blair has begun meeting with Labour MPs in a bid to head off a defeat for his plans to introduce university top-up fees.

With Downing Street describing the vote as "too close to call", the prime minister hopes he can persuade possible dissenters to back his legislation.

He is spending Monday night and Tuesday morning in a last ditch effort to prevent a damaging parliamentary defeat.

Number 10 admitted, however, that by the end of last week the "numbers were not there" to deliver a government victory.

Ministers hope the final push could save the day for the government ahead of what appears set to be the toughest week since Blair took office.

The education secretary had already moved on Monday to give more ground in his battle to push the Higher Education Bill through parliament.

Charles Clarke has promised to include a pledge not to raise the cap tuition fees in the course of this and the next parliament.

The move effectively rules out a rise above the proposed £3,000 levy until at least 2009 and has been included to win round rebel backbenchers who fear ever higher fees.

Number 10 has, however, signalled that there will be no further concessions.

It insisted that the principle of variability - the issue which divides Labour MPs - will not be watered down.

A spokesman for the prime minister said variable fees remained "a very important ingredient" in the legislation.

Many Labour rebels have been won round to plans for variable course costs by a package of sweeteners including the abolition of upfront fees and the restoration of grants for the poorest students.

However a core of critics is refusing to back down on what they see as the development of market forces in education and are threatening to provide the 82 or more votes needed to bring down the bill by voting alongside Conservative and Liberal Democrat MPs.

Clarke has insisted the government will win the vote and said the latest move would convince colleagues that there is no "hidden agenda" to create an "Ivy League" of UK universities.

"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 2006," he told the BBC.

On Monday Clarke was also set to detail to MPs previously announced plans for a review of the impacts of variable fees.

The study would conclude in 2009, three years after the bill would come into effect.

But the latest moves were derided by the National Union of Students.

"The raft of so called concessions has offered very little to students and have certainly not allayed the fears of MPs over the issue of variable top-up fees," said president Mandy Telford.

"'The pledge' that top-up fees will not rise until 2010 is nothing the government has not said before."