They are only students - but could they bring down Blair?

Andy McSmith charts the growth of a rebellion that really does threaten to force the Prime Minister from power

Here is a highly unusual joke about Tony Blair - unusual in that it is a Blair joke, but not at the Prime Minister's expense. Question: How many Labour MPs does it take to change a light bulb? Answer: Tony Blair changes the light bulb - but it takes 150 Labour MPs to sign a motion demanding that he leave the sodding light bulb alone.

That is a view of the political landscape as seen from inside the Prime Minister's bunker, as he limbers up for a struggle that could cost him his job. The present crisis has left observers bemused that one of the cleverest political tacticians of our time, the most successful Labour Prime Minister since Clement Attlee, might be brought down over a question that appears to be so minor.

The political confrontation is not even about who pays for expansion of higher education - the students or the taxpayers. All sides agree that part of the extra cost must fall on those who benefit most. The issue is whether tuition should be allowed to vary from course to course, and university to university.

Tony Blair sees variable fees as essential to allow the top universities to compete internationally, and will not budge from that view. Most backbench Labour MPs see the creation of a free market in university courses as a betrayal of bright children from poor families, and they are not in the mood to back down.

The situation looked dire enough a week ago, when it emerged that 127 Labour MPs had signed a motion calling on the Government to come up with an alternative to variable fees. However, Mr Blair's advisers, and his Education Secretary Charles Clarke, believed that some small concessions and a campaign to win hearts and minds would reduce the rebellion to manageable size.

Mr Blair and Mr Clarke met in Downing Street to agree a plan of action, with Hilary Armstrong, the Chief Whip, and Sally Morgan, the Prime Minister's adviser on relations with the party.

Mr Clarke was in favour of a quick charge towards the sound of gunfire. He wanted to bring the legislation to the Commons before Christmas, fearing that opposition would harden if it was delayed. Mr Blair turned to the Chief Whip for advice, and was told: "If you want a guarantee that the votes are there, all I can say is that I can't give that at the moment." Baroness Morgan agreed.

Mr Blair also agreed. He argued that if a substantial number of rebels were to be persuaded to fall in behind the government, they must be given time and space to do it gracefully. But he told Mr Clarke that he could have an early vote, if he insisted. At this moment, the Education Secretary must have seen a gaping elephant trap opening before him. His position would have been vulnerable indeed if he had ignored the advice of the Prime Minister and the Chief Whip and insisted on an early vote, which was then lost.

He certainly was not going to make a final decision until he had met Gordon Brown. The Chancellor's role in the great top-up fees rebellion is the subject of much animated gossip in the Commons tea rooms. Everyone knows that the former chief whip Nick Brown, who has been co-ordinating the rebellion, is a long-standing friend and ally of the Chancellor. It is widely assumed that they are still in close touch, and that the Chancellor could call off a substantial number of rebels if he wanted to.

However, the face-to-face meeting between the Education Secretary and the Chancellor could not be arranged for Monday night, and did not take place until Tuesday. That delay eliminated any chance of publishing the Bill in time for a vote before Christmas.

When they met, Mr Clarke heard the Chancellor say he backed the policy. That evening, he even delivered a speech defending the principle that students should contribute to the cost of their education, although he avoided the vexed question of variability.

The two ministers also discussed a possible concession to the dissenters - that no student should have to begin repaying tuition costs until he or she was earning at least £20,000 a year, instead of the present threshold of £15,000. But they agreed that would cost too much.

Having ensured that MPs would be given the Christmas break to consider the consequences of another revolt, Mr Blair took his critics by surprise by dropping a heavy hint at his Tuesday press conference that, if he was prevented from getting his policy through, he might resign. What Mr Blair and his advisers hope is that when MPs get out of the Westminster hothouse and reflect, they will decide that their objections to variable fees are not worth the incalculable political cost of defeating a major piece of government legislation.

Even if that is true, resignation threats need to be used sparingly, and this was the Prime Minister's second in eight months. The previous one was when he persuaded the Commons to back him over the Iraq war. Certainly, when Mr Blair faced a meeting open to all Labour MPs on Wednesday, the speakers who opposed his policy outnumbered those who supported him. Later, he faced the House of Commons, where Michael Howard tackled him on the same subject, delivering the memorable line: "THIS grammar school boy will take no lessons from THAT public-school boy on the importance of children from less privileged backgrounds gaining access to university."

Later, Mr Blair's cheerleader Peter Mandelson was overheard on a telephone in the Commons lobby, briefing his newspaper contacts that the Prime Minister had triumphed again. Other Labour MPs gloomily took the opposite view, that it was a rare case of Mr Blair coming off worse in a parliamentary exchange.

Thursday's task was to make sure that the Cabinet was united and committed to battling for the policy. When Mr Clarke made his presentation, he was subjected to some sharp questions from cabinet colleagues, who wanted to know what was wrong - for instance - with the idea being floated by the respected backbench MPs Peter Bradley and Alan Whitehead, to replace variable fees with a flat-rate of £2,500 a year for all courses at all universities. Accounts of the meeting vary. Some of those present say the Education Secretary was shaken by the opposition he encountered, particularly from the Home Secretary, David Blunkett, who opposed variable fees during his four years in charge of Education. He was then responsible for a notorious line in Labour's 2001 election manifesto that read: "We will not introduce 'top-up' fees and have legislated to prevent them."

Mr Clarke's staff say it was a "contructive" meeting and their boss was heartened by the backing he got, not least from Mr Blunkett. What stuck in the mind of many of those present was the brooding silence from Gordon Brown's corner. It appeared that the Chancellor was going to let the debate slip by without saying a word, but he joined in at the end to give formal support to the Prime Minister.

That evening, the Liberal Democrat Treasury spokesman, Alan Law, succeeded where no one else had before him, in getting the Chancellor to say publicly that he is in favour of variable fees. Mr Law intervened in the final stages of the Commons debate on the Queen's Speech to put the question directly to Mr Brown, and received a direct reply - "yes".

Through all this, the revolt just grew. At the start of the week, 127 Labour MPs had signed a motion calling on the Government to offer an alternative to variable fees. On Monday, the number shot up to 144. On Tuesday it rose to 149, by Wednesday 154, and when the Commons broke up for the weekend, it was at 156 (or 157 if you count George Galloway, who was recently expelled from the party).

These figures imply that the Government is heading for a heavy defeat, but the optimists in the Blair camp think that the Prime Minister will get his way. They point out that the motion, which so many MPs have signed, is worded in very general terms and does not commit signatories to voting against the Government.

Tomorrow, Mr Clarke will hold the first of six meetings, to which all Labour MPs are invited, to discuss particular aspects of the policy. He will also explain why the Government opposes the Liberal Democrat idea of a tax on all graduates.

Others are simply bewildered as to why Tony Blair should choose such a high-risk strategy on this particular issue. Some suspect that the Prime Minister, who has had two recent health scares, is in the grip of a political death wish. The vote on university fees is now likely to take place soon after the Hutton report into the death of the weapons expert David Kelly. Some have wondered whether Mr Blair has calculated that a defeat on an aspect of his public sector reforms would provide an honourable pretext for stepping down. But the stakes are also high for the rebels, because if Mr Blair wins this battle, against such heavy odds, he can be very confident of seeing off any other revolts that may arise for the rest of this Parliament.