Al-Qaeda terrorism is not on the same par as the IRA, Prime Minister Tony Blair has suggested.
He said IRA political demands or their previous atrocities could not be directly compared to fundamentalists who carried out the 9/11 US attacks.
It was invidious to make comparisons because "terrorism is wrong", he said.
"I don't think you can compare the political demands of republicanism with the political demands of this terrorist ideology we're facing now."
He was speaking at his weekly news conference on Tuesday
No serious person could ever negotiate on the demands of terrorists who have been using suicide bombers to kill people, said the prime minister.
"I don't think the IRA would ever have set about trying to kill 3,000 people," he said.
"In America, it could have been 30,000 instead of 3,000 and they would prefer that. My entire thinking changed from 11 September - the belief that you have a different form of terrorism."
Mr Blair also said he was continuing to work at ending the IRA's ongoing activity.
He vowed not to "give an inch to terrorism" and said Iraq was no excuse for the London bombings.
He acknowledged Iraq was being used to recruit terrorists, but insisted the roots of extremism were much deeper.
He said 11 September 2001 was a wake up call for the international community, but argued some people "then turned over and went back to sleep again".
The prime minister was speaking to reporters after talks about new terror laws with Tory and Lib Dem leaders.
"We are not going to deal with this problem, with the roots as deep as they are, until we confront these people at every single level," he said.
In response, Ulster Unionist Party leader Sir Reg Empey said he had warned Mr Blair against "creating double standards between terrorists".
"There is no point in using the numbers killed to distinguish between terror groups as the prime minister seems to be implying," he said.
"However, if Mr Blair wants to use a crude stratification process in order to establish a hierarchy of terror, he will find that the number of those murdered and maimed in Northern Ireland is greater."
DUP MP Sammy Wilson said Mr Blair's comments were an "insult to every victim of terrorism".
"Whether a terrorist sets out to murder one person or 100 people, they are a terrorist and no difference should be drawn," he said.