Plans to increase pre-charge terror detention from 28 to 42 days will be dropped from the Counter-Terrorism Bill after a crushing Lords defeat, Home Secretary Jacqui Smith has announced.
In an emergency Commons statement, Redditch MP Ms Smith said the measure will instead be the subject of a separate piece of legislation to be brought before Parliament if required.
Flanked by the Prime Minister, she said: "Some may take the security of Britain lightly - I don't."
To Tory jeers, Ms Smith said she was not prepared to leave Britain "unprotected" against the terrorist threat.
The Counter-Terrorism Temporary Provisions Bill stood ready to be introduced "if and when the need arises".
The emergency late evening statement came just two hours after peers voted by a huge 191 majority against extending pre-charge detention to 42 days.
Ms Smith told the House there was no greater individual liberty than the "liberty of individuals not to be blown up on British streets or in British skies".
She warned: "We face a terrorist threat that is at the severe end of severe."
To further Tory jeers, she said the Commons had voted in favour of the 42-day detention limit but the Lords had taken a different view "despite the considered view of all leading counter terrorism professionals that these powers will be necessary and should be there".
Peers had voted "not to amend, not to strengthen" the legislation but "simply to remove" the provision.
Ms Smith insisted: "My priority remains the protection of the British people. I do not believe, as some MPs clearly do, that it is enough simply to cross our fingers and hope for the best.
"That is not good enough because when it comes to national security, there are certain risks I'm not prepared to take.
"I am not prepared to risk leaving the British people without the protections they need."
Shadow home secretary Dominic Grieve said the Prime Minister's "spin doctors" had prevented Ms Smith from "saying in straightforward terms that she is abandoning 42 days".
He told Ms Smith: "You somewhat demean yourself when you, yet again, come back to this argument that those who oppose the Government's measures are weak on terrorism,
"I have to say to you how profoundly I object to it. We on this side of the House are perfectly prepared to be firm on terrorism, to take resolute measures and, if necessary, pass difficult Bills.
"But they have to be credible, they have to be based on evidence and they must not be put forward in a way that smacks of mere political posturing and gimmicks."
The new Bill, he said, was "one of the most bizarre things I have ever read".
He questioned why the Government could not simply use existing civil contingency powers - as the Tories had suggested - which would achieve the same effect as the new legislation.