Tony Blair is under pressure to clarify advice he received from MI5 on his controversial anti-terror bill.
Mr Blair said he had been advised by the security services
The Tories say the prime minister should withdraw a statement which they say implied security services advised against a "sunset clause" on the bill.
The clause, a 12 month time limit on the new laws, is a major sticking point for the bill's opponents.
Ministers said later Mr Blair was talking about another issue - the burden of proof against suspects.
Liberal Democrat leader Charles Kennedy has suggested the government publish the advice to clear things up.
Those opposing the bill say it has been rushed through without proper debate and say they are determined to fight for the sunset clause.
But asked whether he would reconsider his position on it in the Commons on Wednesday, Mr Blair said he would not.
"I am afraid that we are not prepared to accept either the amendment on the sunset clause or the other amendment voted for by the House of Lords, and which the Conservatives in the House of Lords backed, to change the burden of proof," the prime minister told the Commons.
"That would not be wise; it would be contrary to the strong advice given to us by our security services and our police, and I am simply not prepared to do it."
The Tories said the words implied the security services had told the government not to insert a sunset clause.
But Home Office Minister Hazel Blears later said that the advice referred only to the burden of proof issue, not the sunset clause.
Conservative leader Michael Howard has asked Mr Blair to withdraw the statement, which he said was "not accurate".
He told the BBC Mr Blair had compounded damage his credibility suffered over the Iraq war, after which Mr Blair apologised for mistakes in pre-war intelligence about weapons of mass destruction.
And Liberal Democrat MP David Heath told the Commons: "The clear moral from this is never believe a thing that the prime minister says he is told by the security services."
But the prime minister's official spokesman defended his words on Friday, saying security services wanted the new law and they wanted certainty.
But, he added, they would not comment on specific parliamentary moves.
Mr Blair told the BBC: "The security services and the police are advising us that we need these powers to defeat those who are planning and plotting terrorist activity in our country.
"They want them on the statute book... they don't want them put on and then a signal sent to the terrorists: 'Well, if you wait a certain period of time these powers are going to evaporate or disappear'."