MI5 has not "directly" asked the government to extend the time limit for holding terror suspects without charge, to 42 days, Jacqui Smith has said.
But the home secretary told BBC One's Andrew Marr Show the security service had been "clear about the growing scale of the threat" to the UK.
She added that she hoped the government would not lose a crucial Commons vote on the issue on Wednesday.
The Conservatives, Lib Dems and up to 30 Labour MPs oppose the plan.
The opposition parties argue the proposed pre-charge detention limit will infringe civil liberties, but ministers argue it is necessary to deal with increasingly complex terror plots.
Ms Smith said extending the limit for terror suspects from 28 days to 42 days was a "safeguard, not a target", and that it was a "reasonable maximum".
It's completely against the traditions and ways of this country
Boris Johnson, London Mayor
It would allow suspects "through the criminal justice system in the most effective way", she added.
Questioned as to whether MI5 had asked for extended detention powers, Ms Smith said: "No, not directly, but nor did they ask for the extension from 14 to 28, nor did they ask for the extension from 7 to 14."
Asked whether the 42-day plan, part of the Counter-Terrorism Bill, would get through Parliament, she replied: "I certainly hope it does, because I believe it is the right thing to do."
The Tories have previously said MI5 is not privately pressing the case for 42 days.
Shadow home secretary David Davis told BBC One's The Politics Show: "The question I have to ask every time is will this save lives?
"Will this actually achieve what we are trying to achieve, or will it do the opposite? In my view, very plainly, it will do the opposite."
Liberal Democrat home affairs spokesman Chris Huhne said: "MI5, the director of public prosecutions and senior police officers think that this is an unnecessary extra power.
"The reality is that the 42-day proposal is entirely arbitrary."
Prime Minister Gordon Brown has written to all Labour MPs, seeking their support for the plans.
He said new safeguards would mean suspects were held for six-weeks only during "a terrorist crisis situation".
But the combined opposition to the plans raises the prospect of Mr Brown suffering his first Commons defeat since becoming prime minister last year.
Ministers are hoping the fears of potential rebels have been assuaged by an address to the Parliamentary Labour Party by Ms Smith last week, in which she proposed new safeguards to prevent arbitrary use of the new powers.
The home secretary said there was "absolutely not" an agreement to provide £200m of extra funding to the administration in Stormont in exchange for the backing of the Democratic Unionist Party's nine MPs.
She added: "This is not about doing deals. This is about doing the right thing by the country, and this country's security."
Mr Brown, who has a working majority of 65 in the Commons, has made it clear that he does not regard Wednesday's vote as a matter of confidence in his premiership.
Senior police officers have backed the 42-day plan, but the director of public prosecutions Sir Ken Macdonald said that Ms Smith's safeguards had made no difference to his belief that the change was unnecessary.
Parliament's Joint Committee on Human Rights dismissed the new safeguards as "inadequate" and said the case had not been made for justifying the longer period.
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