The ministers will explain the fee policy to MPs

The education secretary held the first of several 'seminars' with Labour backbenchers to make the case for the proposals.

Ministers hope to try to avert a Commons defeat on a policy which Tony Blair has staked his authority.

But the BBC has learned that the bill will not include details of issues like maintenance grants for poorer students.

Those will be published in secondary legislation at a later date - a fact that is unlikely to quiet discontent among the government's opponents.

Variable fees

It would take 81 of 150 declared Labour opponents to vote with opposition MPs for the tuition fees bill to fail.

The top-up fees policy will allow universities to charge variable amounts for tuition, payable by students once they graduate and get a job.

The principles of these reforms are right ... Tony Blair will win this vote Gordon Brown Chancellor

It was the issue of variable fees that was highlighted by one Labour opponent to the plans, MP Martin Salter, who expressed the fear that the move would create a market in higher education.

"That will create in the view of many of us the opportunity for children or young people from better-off backgrounds to attend the elite universities at the expense of children from less well-off backgrounds," he told BBC News.

Mr Clarke insisted that his plans pulled out "many of the key fairness principles of a graduate tax" and he said he welcomed the opportunity to have a debate with the Parliamentary Labour Part.

"Both systems require graduates not students to pay contributions towards their higher education," he said.

"Both systems link repayments to earnings, so payments are always based on what the graduate earns, not on what they owe.

"Both systems mean that if a graduate doesn't work they don't pay.

Guaranteed income?

"Both systems never charge a real rate of interest - the government pays the cost of borrowing under both proposals."

He said the difference was that the government's plans provided a guaranteed source of income for universities "independent of the government".

But the government received a warning earlier in the day from ex-cabinet minister Robin Cook who had a message for the prime minister.

"I would say to Tony Blair there is nothing dishonourable in canvassing opinion and then discovering the majority against you," he said during a speech in Glasgow.

The former foreign secretary and leader of the Commons said he hoped Mr Blair would not "push this deeply divisive issue to a vote".

'Defeat not the object'

"The object of those of us opposed is to halt tuition fees, not to defeat the prime minister," he said.

On Sunday Chancellor Gordon Brown told the BBC he has no doubt the government would win the battle over top-up fees.

There has been speculation that Mr Blair would quit if the fees vote is lost, reigniting the chancellor's alleged Labour leadership hopes.

However, Mr Brown insisted: "The principles of these reforms are right... Tony Blair will win this vote."

Meanwhile, Higher Education Minister Alan Johnson said ministers were prepared to look at concessions over the plan.

Threshold demand

The proposals at the moment would see graduates starting to repay tuition fees once they start earning �15,000-a-year.

Some Labour MPs wanted the threshold lifted to �18,000 or �20,000.

But Mr Johnson said the government would not compromise on variable fees, which would allow different universities to set different fees for different courses up to a maximum of �3,000 a year.

Mr Blair argues the top-up fees measure is essential if the government is to achieve its target of getting 50% of young people into university.