Tony Blair, Gordon Brown and Charles Clarke have hit the media trail to defend plans to introduce tuition fees.

Ahead of Tuesday's crunch Commons vote the prime minister warned his opponents that Britain's universities could become "second class" if his controversial bill was defeated.

Education secretary Charles Clarke also revealed another concession ahead of Tuesday's crucial vote on the Higher Education Bill.

In a final series of meetings with backbenchers over the weekend he pledged that the £3,000 cap on university top-up fees will not be raised for two parliaments.

That is a freeze on fees until at least 2009 - three years after the charges come into effect.

But Clarke has refused to give ground on variable fees which is the main concern of Labour rebels.

"This really is the time of choice. They have got to decide where they are. I am confident that we will win the vote," he told the Independent on Sunday.

Allowing universities to charge more for popular courses is a vital part of the government's plans and is "non-negotiable".

The prime minister used an interview with the Observer to claim that Britain's universities would face massive problems if MPs rejected his plans.

"If we don't get the extra investment in, in the future we will have a second-class university system," he said."If we don't put in place a new system of university finance, fewer people will be able to go to university, and fewer people will have the quality education that they want,"

Blair also claimed some opponents were using the top-up fees issue as a vehicle to launch an assault on his leadership.

"The odd thing I find is that there is no great heat in this argument, most people actually accept we are trying to do the right thing for the country. There may be other reasons for people voting," he said.

Adding to the political impetus, the chancellor told GMTV that Blair had his full backing for a "sensible, radical reform".

He also predicted that many of the rebels would change their minds "over the course of the next day".

Liberal Democrat leader Charles Kennedy claimed on Sunday that most Labour MPs were closer to his party's position but were constrained by loyalty.

"There is obviously a very significant Labour rebellion and I think most people actually agree with our Liberal Democrat position, which is that if you believe in 'education, education, education', then it should be funded out of general taxation," he told the BBC.

"That is socially fair and it is also in the long-term interests of the country, so we will be voting accordingly. I hope this Bill is defeated because I think that the scandalous level of debt that so many students now find themselves in is really working against their direct interests and against the long-term interests of all of us who depend on a well-educated society."