Up to 30 Labour MPs are reported to be against the proposal. Defeat would be humiliating for Brown and seriously undermine his leadership, dented by poor election results, a slowing economy and poll ratings at an all-time low.
"We are confident in the argument but the vote is very, very tight," said Brown's spokesman.
The government wants to extend the maximum time police can hold and question terrorism suspects before having to charge or release them to 42 from 28 days. It says the measure would be used only for "grave and exceptional terrorist threats".
Victory would give Brown, who has a majority of 65 in the Commons, much-needed breathing space but a significant Labour revolt could leave the government dependent on support from minority parties.
Attempts by Tony Blair to extend the detention limit to 90 days in 2005 ended in failure, dealing Brown's predecessor his first parliamentary defeat as prime minister.
"It is entirely unpredictable," former cabinet minister and Labour rebel Frank Dobson told BBC radio.
Opponents of the bill say it is an infringement of civil liberties and a threat to police attempts to win "hearts and minds" in the fight against terrorism. They say most other countries have detention periods shorter than 28 days.
Dobson said some MPs would have been convinced by government concessions while others would end up backing Brown in his biggest parliamentary test out of loyalty to the party.
The government has offered concessions on strengthening parliament's say on when the extended limit could be used and there were media reports on Wednesday of further measures to placate restive Labour MPs.
One concession reported by the media was a possible payment of 3,000 pounds for every day suspects are held above 28 days and later not charged. Brown's spokesman dismissed the figures reported on Wednesday.
Brown has refused to concede on the fundamentals of the plans which he says are supported by the public. A YouGov survey published in the Daily Telegraph showed 69 percent of Britons would support the extension.
"Our first duty is the protection of national security. We fail in our duty if we do not take preventative measures," Brown told parliament on Wednesday.
"I say in sorrow rather than anger 'it is no use opposition for opposition's sake'. We have to take no risks with security."
To guarantee a win, Labour is likely to need either absences from at least some minority parliamentarians or support from the Democratic Unionists. In 2005, they voted against the plan to hold suspects for up to 90 days.
