Home Secretary Jacqui Smith has announced plans to extend the period that terrorism suspects can be held without charge for up to 42 days.
In an attempt to win over critics, it is planned to give MPs a role in any decision to let police hold suspects for more than the current 28-day limit.
Ms Smith said 42 days would be needed only in "exceptional circumstances".
Ministers have come up against strong opposition from Tories, Lib Dems and some Labour MPs over terror detentions.
'Murder and mayhem'
Ms Smith told the BBC there was a "serious and consistent threat from terrorism".
She added: "In order to ensure we prosecute people who want to cause murder and mayhem on our streets, we may well need to hold them longer to do that."
Ms Smith also said: "We need to legislate now for the exceptional circumstances that there might be in the future."
The home secretary will have the power to set a 42-day limit.
Ms Smith said: "To emphasise that the higher limit is exceptional, it must be approved by Parliament within 30 days."
Ministers recently proposed extending the terror detention limit to 56 or 58 days.
The 28-day limit was itself a compromise, after former prime minister Tony Blair failed to convince Parliament to increase it to 90 days in 2005.
Apart from ministers, the government-appointed reviewer of terror laws, Lord Carlile, has said he thinks there is a case for extending the detention limit.
But civil liberties groups have accused ministers of abandoning consensus and playing politics with terror laws.
The Commons home affairs select committee, which meets to consider the issue later, is also believed to be unconvinced.
Terror 'arms race'
Shami Chakrabarti, director of civil liberties campaign group Liberty, accused the government of abandoning attempts to build a consensus on the issue.
"It seems more like politics than policy-making to me. I think it is a real mistake as I've never seen such good will in the House of Commons," she said.
"I believed there was a unique opportunity to end the arms race on terror laws, to actually achieve something that would last beyond just one government."
The Conservatives and civil rights group Liberty have argued that current powers to call a national emergency are sufficient when faced with a case that requires going beyond 28 days.
Shadow home secretary David Davis said: "They [the government] are making a proposal for something they still have not proved necessary.
"They have lost the argument to further extend pre-charge detention beyond 28 days again and again."
Liberal Democrat home affairs spokesman Nick Clegg said: "The government is tying itself up in knots in an ever-more desperate attempt to sweeten the bitter pill of this proposal for its own backbenchers.
"Frankly, what the government is now proposing is not easy to understand."