The government says it is sticking by plans under which terror suspects could be kept under house arrest.
Policies on detaining terrorism suspects have proved controversial
The prime minister is to meet the Tory and Liberal Democrat leaders on Friday, to try to get an all party agreement on controversial anti-terror powers.
Both have signalled their opposition to the plans and there has been speculation concessions will be made.
But the prime minister's spokesman said on Monday that "extreme measures" were necessary to deal with the threat.
He said the meeting with Michael Howard and Charles Kennedy would be a "serious discussion about serious issues".
"The government has not put forward its proposals lightly," he said.
"The control orders would allow a range of options, and at the top end, there are what everybody agrees are extreme measures, but extreme measures to deal with extreme circumstances."
The new powers are being sought after the House of Lords ruled the previous policy of detaining foreign suspects without trial was unlawful.
Home Secretary Charles Clarke suggested "control orders" as an alternative, which include restricting the movement of terrorist suspects through electronic tagging, curfews or house arrest.
The government wants the new powers brought in as quickly as possible.
But it may run out of time in this parliamentary session unless they have the support of the opposition parties.
There has been speculation that the government would offer concessions on the controversial policy, in order to get the measures in place quickly.
Newspaper reports suggested there were concerns from MI5 and the police that protesters could target suspects' homes.
On Sunday Mr Clarke denied he would be "back tracking" on the policy, but added: "I have never actually used the phrase 'house arrest'."
"What I said in the House of Commons was that we would be introducing a regime of control orders up to and including the ability to restrict people on the premises where they live and that is what we will be doing."
The prime minister's spokesman also said there would be no change in the government's opposition to using phone-tap evidence in court.
Some say it would allow charges to be brought against suspects who would otherwise be held without trial.
But the spokesman said: "Superficially it may seem an attractive thing, but it is not true that it would make much difference and it would do so by potentially putting at risk sources of intelligence."