PM's biggest risk is that he will face Commons defeat for first time

The Terrorism Bill reaches its report stage in the Commons this afternoon when proposed amendments are considered. Most of its measures carry broad support, but some have proved controversial, including plans to outlaw "glorification" of terrorism and the proposal to lock up terrorist suspects without charge for up to 90 days.

Last week the Government won a vote on the "glorification" proposal by just one vote after 34 Labour MPs rebelled. In the face of near-certain defeat, Charles Clarke was forced hours later to withdraw the detention proposal.

So what will happen today?

After days of arm-twisting of Labour MPs, even wheeling in the police to bolster his case, Tony Blair will press ahead with the 90-day plan. His only concession has been to offer a so-called "sunset clause" guaranteeing that the measure will have to be approved again by MPs in a year.

The first Commons vote on the detention period will be on the 90-day proposal.

There will be no further votes if ministers win. But if the Government loses, or if Mr Clarke has to withdraw the proposal again because he fears defeat, MPs are likely to vote on a detention period of 28 days. This compromise is supported by the Tories, Liberal Democrats and dissident Labour MPs.

If that is defeated, MPs would vote on a proposal from loyalist Labour MP Janet Anderson to increase the detention period to 60 days.

Were that also voted down, the Bill would be in chaos.

What are Mr Blair's tactics?

His refusal to back down over 90 days has surprised the Home Office. But the Prime Minister is determined not to appear to be in retreat on an issue he has championed. He believes the mood among his backbenchers is shifting in his direction and he has told them the issue could produce a party political dividend.

Mr Blair is convinced the public strongly supports tougher measures against terrorist suspects and that it will do no harm to have the Tories and Labour on different sides of the argument.

What are the risks he is running?

The biggest, obviously, is that for the first time in eight years he will be defeated in the Commons as MPs throw out the 90-day proposal. Such a humiliating reverse would ignite fresh speculation over his future.

Victory carries more subtle risks. He will have forced through contentious legislation in the teeth of fierce opposition, perhaps relying on Tory votes to scrape though. That outcome would hardly shore up his standing with MPs and party activists.

Will the Government win?

The bookies are offering odds of 1/2 that Mr Blair will get his way, but the numbers remain desperately close. The chances have improved slightly because some Labour critics have changed position and a handful of Tories are preparing to back the Government.

What happens then?

The Bill will get its third reading in the Commons tomorrow, before going to the House of Lords. The Government had hoped to push the Bill on to the statute book by Christmas, but that is highly optimistic. The Lords is expected to throw out any increased detention period and the "glorification" proposal, returning an emasculated Bill to MPs.