Brown wants to extend the maximum time limit that terrorism suspects can be held without charge to 42 days, from 28 days, but opposition parties and sections of the Labour Party oppose the move on civil liberties grounds.
Defeat at a vote on Wednesday would seriously undermine Brown's leadership at a time when his popularity has plummeted so far that some Labour MPs are questioning his position.
Brown aides privately think the government has done enough to win over critics after it offered new human rights safeguards although publicly they say the vote is by no means won.
In a boost to Labour rebels, parliament's joint committee on human rights on Thursday said the proposed amendments to the bill were negligible and that the move would still violate the European Convention on Human Rights.
"While of course I welcome any new safeguards, no matter how minor, they do not answer the main point that we have seen nothing that would justify the extension beyond 28-days maximum detention," said Labour MP Andrew Dismore, chair of the committee.
"The proposed extension to 42 days would almost certainly not be lawful," he added in a statement published alongside the committee's report.
The two main opposition parties oppose the 42-day pre-charge detention plan.
Even if Brown wins the vote in the Commons, the bill is expected to run into opposition in the Lords.
Brown aides argue the legislation does not violate human rights laws.
"We are very confident of the strength of our argument but any suggestion that the votes are already in, already won, are wide of the mark. We're not at that point yet," Brown's spokesman said on Wednesday.
In one of its amendments, the government said the 42-day detention powers would only be used against a "grave exceptional" threat - envisaged as something on the scale of suicide bombings in London that killed 52 commuters in 2005.
The human rights committee argued that having a government minister declare that such a threat existed to parliament "is not, in reality, much of a safeguard, at least without some meaningful opportunity for that assertion to be tested by independent scrutineers, whether in parliament of the courts."
Brown has sought to play down suggestions that he would have to resign if he lost Wednesday's vote, although defeat would prompt fresh speculation about his leadership and his ability to lead Labour to victory in the next election, due by 2010.
