Jacqui Smith is to press ahead with moves to hold terror suspects for up to 42 days without charge despite the growing prospect of defeat on the measure.
Gloomy Labour whips fear a humiliation for the Government on the issue despite great efforts by the Home Secretary to talk round her Labour critics.
There has been speculation at Westminster that the Government would be forced to drop the proposed increase from 28 to 42 days.
Concessions over the detail of the detention plans are possible before the crucial Commons vote on the Counter-terrorism Bill, which is expected in late May. But Ms Smith made clear yesterday that she was not backing off.
She told police chiefs: "You will be all too aware of the importance we must now place on acting quickly – often in the very early stages of investigations and before evidence has been gathered – to disrupt terrorist activities. This is the rationale for the additional legal powers your senior officers have requested and which I am now taking through Parliament."
Home Office sources insisted Ms Smith would not "cave in" on a "real world" issue and was confident of winning a Commons majority when it came to the crunch. A survey of Labour MPs by The Independent four months ago identified at least 38 preparing to vote against the increase, enough to wipe out Gordon Brown's lead of 67. The Government could be in the unlikely position of having its fate in the hands of nine Democratic Unionist MPs.
Ms Smith also set out plans to recruit more than 300 extra police officers to combat the terrorist threat.
The Tories claimed her announcement breached rules banning the Government from making political announcements in the period before elections. Voters go to the polls in English and Welsh councils, as well as for the London Mayor and Assembly, on 1 May.