A new round of cross-party talks is to be held as ministers "seek consensus" for extending the 28 day limit for holding terror suspects without charge.
Gordon Brown says he wants to build a consensus on the issue
The Home Office said it had "consulted widely on options", but no decision on any changes had been taken.
Ministers are believed to be drawing up a range of safeguards to allow suspects to be detained for up to 58 days.
But David Davis, for the Conservatives, said there was not a "an ounce of evidence" for extending the limit.
Under the government's proposals, detention without charge beyond 28 days could only be triggered in exceptional circumstances agreed in advance by Parliament.
This would included cases where there are "multiple plots, or links with multiple countries, or exceptional levels of complexity", the Home Office said in a statement.
It would also require the home secretary's agreement and the extension of the powers would be time limited. There have already been assurances about greater judicial approval and parliamentary scrutiny.
Ministers say the opposition parties have already accepted in principle that suspects could be held for up to 58 days under existing emergency powers.
As a result, Downing Street said earlier, what was needed now was "a technical discussion" about the circumstances for an extension and the necessary safeguards.
Ministers do not, at this stage, want to propose a new maximum time limit and may not do so until they are confident that they can secure a parliamentary majority for it.
In the past the prime minister has suggested doubling the existing limit of 28 days, which would take it up to a maximum of 56 days.
But BBC Political Editor Nick Robinson said the government hopes to move the debate away from the number of days and on to the criteria for what they insist will be a power used only in rare and special circumstances.
Shadow home secretary David Davis said the Conservatives would support the use of temporary emergency powers in exceptional circumstances, such as a large number of simultaneous terror plots being probed by police.
But he said the government appeared to be proposing a "permanent undeclared state of emergency" and the Conservatives could not support such a move.
Lib Dem home affairs spokesman Nick Clegg said: "How does Gordon Brown seriously think he can forge a national consensus on such a vital issue without any new evidence and with utter disregard for the strong opinions of those who believe it would be a step too far?"
The government-appointed reviewer of terror laws, Lord Carlile, said there were "going to be a few cases in which more than 28 days is going to be needed for proper investigations to be carried out".
He told BBC Radio 4's The World at One he expected to see "firm proposals" from the Home Office "by the end of tomorrow(Friday)".
But he cautioned against using emergency powers under the Civil Contingencies Act, which he said would be a "very bad idea".
"We don't have a culture in this country, Musharraf style, of declaring states of emergency," he added.
But he said that with better judicial scrutiny "very few people, perhaps one or two in the next two or three years would be held for more than 28 days and quite a lot of people who have been held for up to 27 days and some hours would be held for a rather shorter time than has been the case to date".
The issue of how long terror suspects can be detained without charge led to Tony Blair's first Commons defeat as prime minister, when he sought to increase the limit to 90 days.