Plans to extend the limit to 90 days were defeated in 2005
Government plans to extend the limit to hold terror suspects without charge to 42 days are facing mounting criticism from both opposition and Labour MPs.
Keith Vaz, Labour chairman of the Commons home affairs committee, said ministers did not have enough support in parliament to carry the plans.
It comes after the home secretary described the terrorism threat facing the UK as "severe" and "growing".
The Tories and Lib Dems say increasing time limits will not help fight terror.
In a News of the World interview, Home Secretary Jacqui Smith said 30 terror plots were being investigated and police needed to be able to detain suspects for longer.
Ms Smith said: "We now face a threat level that is severe. It's not getting any less, it's actually growing.
"There are 2,000 individuals they are monitoring. There are 200 networks. There are 30 active plots.
"That has increased over the past two years. Since the beginning of 2007, 57 people have been convicted on terrorist plots...
"We can't wait for an attack to succeed and then rush in new powers. We've got to stay ahead."
In November, Jonathan Evans, MI5's director general, spoke of 2,000 people posing a threat to the UK - the same number quoted by the home secretary. But he went on to say that the number had not peaked.
The Counter Terrorism Bill passed its first Parliamentary hurdle earlier this month when it was given an unopposed second reading but it now goes to committee stage where MPs will scrutinise the detail.
Under the new proposals, the home secretary would be able to immediately extend the detention limit of a suspect from 28 to 42 days, as long as it was supported by a joint report by a chief constable and the director of public prosecutions.
The extension would then have to be approved by the Commons and the Lords within 30 days. But if either House voted against it, the power would end at midnight on the day of the debate.
Keith Vaz questioned why 42 days was needed if the government now agreed to post-charge questioning.
"If they agree the three points set out in the Home Affairs Committee report, I will support them. That would mean the vote is much closer.
"If the committee votes with the government it would bring about 20 waverers on-side. If not, they're likely to lose."
The proposals are supported by some senior police officers but, if passed, could face a court challenge from the Equality and Human Rights Commission.
In 2005, the government was defeated over proposals to extend the limit to 90 days.
Tony Blair's former attorney-general, Lord Goldsmith, reiterated his opposition to the 42-day limit and told Sky News ministers were embarked on a wrong course.
"I still hope that they may see that ultimately it doesn't make sense to pursue this, and they will get huge support from a lot of other people - from me in particular - for all the other steps that they want to take in order that we have a tough, firm but fair policy on prevention of terrorism."
Health Secretary Alan Johnson told the BBC he was "absolutely certain" the government would get the policy through the Commons.
He said the argument about whether to extend the time would "reverberate around Parliament" in coming weeks, but pledged the government would listen to MPs' views.
Conservative security spokeswoman, Dame Pauline Neville-Jones,said: "What you need if you've got an increase in plots is the right quantum of resources for both the police and the intelligence services to track and disrupt the plots - and that's a question of bringing resources to bear."
Lib Dem home affairs spokesman Chris Huhne said: "We have a 92% conviction rate on terrorist offences. For heavens sake, that's far higher than on ordinary criminal cases.
"So what on earth is the government on about in saying that we need to go further than any comparable country has anywhere else in the world?"