Reports on Wednesday suggested that Brown would have to rely on the support of smaller parties to win the vote.
In a sign of the closeness of the vote, David Miliband, the foreign secretary, has been forced to cut short his visit to the Middle East to return to London to take part in the vote.
Defeat would increase pressure on the prime minister, whose party is close to record lows in opinion polls and has been hit by recent defeats in local elections, the London mayoral contest, and a by-election in a party stronghold.
A poll published in the Daily Telegraph newspaper put public backing for the proposals at 69 per cent, with 24 per cent opposed, while another survey in The Times newspaper on Tuesday had 40 per cent fully backing an extension with a further 33 per cent supportive, provided there were strong safeguards.
Arguments being used to support the increase include the view that police require more time to investigate increasingly sophisticated plots, often with an international dimension.
Opponents of the controversial "counter-terrorism" bill include the parliament's oversight committee on human rights, civil liberties groups and senior legal figures.
Rizwaan Sabir, a postgraduate student at Nottingham university who was held without charge for six days in May under terrorism legislation, told Al Jazeera that the plans to hold suspects longer than 28 days have nothing to do with protecting the public.
He said: "This is power politics being played out in the houses of parliament. It makes me think about what the government's agenda is… is it to maintain the status of a police state over a certain group?
"If they're trying to stop the radicalisation of Muslims the way to do that is not by locking away innocent people for 42 days... That will exacerbate the problem."
Sabir said that he was subjected to psychological torture during his detention.
"It was the most degrading, dehumanising encounter I've ever experienced.
"They [the police] ask you the same questions in different words and you're deemed to be guilty as soon you're arrested… which is a complete violation of the rule of law in this country," he said
Sabir was arrested after police discovered he had downloaded an al- Qaeda training manual as part of his PhD research into radical Muslim groups around the world.
Lord advocate, Elish Angiolini, Scotland's chief prosecutor, has also joined the government's critics, echoing the views of Ken Macdonald, the director of public prosecutions in England and Wales.
Lord Angiolini told the Herald newspaper in Glasgow that she was "not aware" of any cases in Scotland where an extension beyond 28 days would have been necessary.
|Tony Blair failed to increase the detention |
limit from 14 days to 90 days [AFP]
Meanwhile, Martin Scheinin, the United Nation's special rapporteur on human rights and counter-terrorism, urged Brown to think again.
"I appeal to the [British] government to withdraw the bill or to postpone taking a definite decision on it," he said in Geneva.
Tony Blair, Brown's predecessor, tried and failed to increase the pre-charge detention limit from the then-maximum 14 days to 90 days after the July 2005 suicide attacks in London.
The attacks on London's public transport network killed 56 people, including the bombers themselves.