Police can question terror suspects for up to 28 days before charge
Downing Street insists it is pushing ahead with attempts to extend terror detention without charge to 42 days.
Gordon Brown's official spokesman said the prime minister believed this was the "right thing to do".
The BBC understands ministers have warned it would be "politically suicidal" to try to force the measure through against the wishes of peers.
No 10 declined to answer "hypothetical" questions about whether the measure would be dropped if peers reject it.
The plan scraped through the Commons and is due before the House of Lords next week, where it is expected to be defeated.
'Let it go'
Governments can use the Parliament Act to force through measures even if peers reject them - but it is a controversial, time-consuming and rarely used process.
The Times reported on Monday that the government had decided against using the act.
But Downing Street insists it will not shelve plans to extend the detention limit to 42 days, which are opposed by the Conservatives, Lib Dems and some Labour MPs.
Liberty director Shami Chakrabarti told the BBC the measure would be defeated in the House of Lords, but added: "The question is will the government then let this go with dignity ... or will it press ahead for another vote on ping pong as it's called in the House of Commons?"
Andrew Dismore, the Labour MP who chairs Parliament's human rights committee, told BBC Radio 4's World at One he wanted the government to look at "other alternatives" such as existing emergency powers in the event of a "genuine threat to the life of the nation".
'Convoluted and unworkable'
Shadow home secretary Dominic Grieve suggested there had been some "extraordinary dithering" by the prime minister.
"These proposals are unjustified, they are unworkable and they are unnecessary and I suspect every government minister knows it," he said.
"The prime minister should come to a decision and drop it but I suspect he's very reluctant to do it because it would have to do with his standing."
In June, MPs passed the proposal by 315 votes to 306, despite a rebellion by 36 Labour backbenchers - after nine DUP MPs backed the government. The prime minister denied opposition claims they had effectively been bought off with inducements.
Several amendments were made before the Commons vote to win over Labour MPs.
Scotland Yard's former anti-terror chief Andy Hayman told the Times earlier he believed the concessions had made the scheme "bureaucratic, convoluted and unworkable".
Liberal Democrat home affairs spokesman Chris Huhne added: "As we see from former counter-terrorism chief Andy Hayman's comments, even many of those in the security services who support longer detention without charge are opposed to this fudged and deeply flawed bill."
Former MI5 head Baroness Manningham-Buller has criticised the plan as unworkable and last week Europe's human rights body expressed concerns about suspects being detained in police cells for longer than 14 days.
The government argues there may be occasions when a suspect has to be held for longer than 28 days before a charge can be brought, because of the increasing complexity and scale of the terrorist threat.