The government meanwhile summoned home early two senior cabinet ministers from overseas to bolster its chances of winning the vote amid opposition to the proposals from both inside and outside the ruling Labour Party.

"Let's send out a signal from this house that when it comes to defeating terrorism we are going to give the police the powers they need and back them," Blair told the House of Commons ahead of the vote.

A rejection of the measures -- including one that extends the period for which suspects can be held without charge from the current 14 days to 90 days -- would be Blair's first legislative defeat since coming to power in 1997.

The legislation also calls for a ban on those who glorify terrorism, sell extremist books, receive or provide terrorist training, or prepare to commit attacks.

Blair said longer detentions would help police glean more evidence and build stronger cases.

"We're not talking about large numbers of people, but they may be the crucial difference in saving this country from terrorist acts than not," the prime minister said.

Police called for longer detention powers following the July 7 bombings in London because of the greater use of encrypted computers, as well as language difficulties inherent in combating international terrorist networks.

In the context of his new appeals, Blair said the country was faced with an unprecedented threat of mass casualties, a darker threat than the terrorism over the previous three decades carried out by the Irish Republican Army.

Since the attacks on July 7 by four presumed Muslim suicide bombers who killed themselves and 52 commuters in London, the threat of more attacks looms, he warned.

"In the last week, we have learned that since the 7th of July two further terrorist plots have been prevented in this country," Blair told lawmakers without elaborating.

Blair faced a rebellion last week by Labour lawmakers seeking to set the detention period at 28 days. The Conservative and Liberal Democrat opposition parties as well as human rights groups have also denounced the detention plans.

As a sop to critics, Blair's interior minister Charles Clarke has proffered a "sunset clause" which would see the legislation expiring in 12 months unless renewed by lawmakers.

Clarke also offered the stipulation that a suspect's detention would be reviewed by a High Court judge every seven days and a code of practice to govern the treatment of those held.

To ensure the votes necessary for the legislation, the government recalled Foreign Secretary Jack Straw from Russia and finance minister Gordon Brown from Israel.

Brown told reporters in Tel Aviv he would vote for the measures which he deemed crucial to national security. "It is important that I maximize the Labour vote and encourage others to do so," Brown told Sky News television.

The Foreign Office in London said Jack Straw would still have time to attend meetings involving his French, German and Russian counterparts before leaving Moscow.

Defense Secretary John Reid was returning from a visit to the United States in time to vote for the law, though a defense ministry spokesman said his trip had not been cut short.

Wednesday's vote is seen as a key test of Blair's authority, with the Labour rebellion last week prompting media speculation that he may be forced to step down long before serving out what he said will be his third and last term.

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