Baroness Manningham-Buller tells the Lords why she is against the plans
The former head of MI5 has dismissed government plans to extend the time terror suspects can be held to 42 days as not "workable".
Baroness Manningham-Buller, who stepped down from the role last year, told peers she disagreed on a "practical basis as well as a principled one".
But the government said terror attacks were a "clear and present danger".
The House of Lords is widely expected to block the plan, which passed through the Commons by just nine votes in June.
The government wants to extend the maximum period a terror suspect can be detained without charge from 28 to 42 days - it says this is needed to deal with increasingly complex plots.
But a rebellion by Labour MPs in the Commons meant the measure was passed there only thanks to DUP MPs' backing.
Prime Minister Gordon Brown's plan looks set to face an even tougher battle in the Lords, where the combined total of Tory and Lib Dem peers outnumber Labour members.
We face an unprecedented terrorist threat
Security Minister Lord West
Lady Manningham-Buller, in her maiden speech to the House of Lords, said: "I don't see, on a principled basis, as well as a practical one, that these proposals are in any way workable."
She added that a "broad cross-party consensus" on fighting terror was needed.
Lady Manningham-Buller also said it was essential there was a "balance between the right to life", "the fact that there is no such thing as complete security" and "the importance of our hard-won civil liberties".
She added that "therefore, on principle, I cannot support 42 days' pre-charge detention in this bill".
Lord Goldsmith, who was attorney general when Tony Blair was prime minister, also opposed the plan, saying it risked "giving away the very freedoms that terrorists are trying to take from us".
He told peers: "We don't take away freedoms that people, and our ancestors, have fought for without very good cause..."
Former Lord Chancellor Lord Falconer said he would oppose the plans "root and branch".
He said: "I'm absolutely clear that there's no advantage for fighting terrorism that will be obtained from extending detentions to 42 days."
He added that an extra 14 days available to question suspects and gather evidence would "make no difference" to the police, and that to say so was a "ridiculous assertion".
For the government, Security minister Lord West said: "We face an unprecedented terrorist threat... There's a clear and present danger to our population."
He added: "The terrorists we face are more ruthless than those we faced in the past."
But shadow security minister Baroness Neville-Jones said of the 42-days plan: "It represents yet another attack, on the part of the government, without justification, on fundamental democratic rights and freedoms that have underpinned our society for centuries."
This draconian proposal will be defeated in the House of Lords
Lord Thomas Liberal Democrats
The Conservatives and Lib Dems say they will try to defeat the 42-day measure when the bill reaches its detailed parliamentary stages, possibly after the long summer recess.
Lord Goldsmith, who was the legal adviser to Tony Blair's government after the terror attacks on London and in the run-up to the Iraq invasion, is due to speak in the debate and has made no secret of his opposition to the plans.
He has said "the case has not been made" to extend the limit on holding terror suspects, and he could not support it.
He also argued that extending the pre-charge limit to 42 days would send a message to Muslims "that we are down on them".
Lord Thomas, who speaks on attorney general issues for the Lib Dems, said: "Holding someone without charge for 42 days adds nothing of value to the investigation, prosecution or conviction of terrorists.
"The government's proposals do not have serious support amongst senior police officers, ex-government law officers, the director of public prosecutions, the legal profession or, indeed, anyone who knows anything about the criminal justice process.
"Gordon Brown's clinging to the shreds of his discredited policy in the hope that he can pass off this ineffective and unprincipled stance as somehow 'tough' on terror.
"This draconian proposal will be defeated in the House of Lords."
Attempts to extend the terror detention limit to 90 days in 2005 ended in Mr Blair's first Commons defeat as prime minister.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.