According to him, radical Islamists believe that whatever is done in the name of their cause, including the use of chemical, biological or nuclear weapons, is justified.
"After 11 Sep, rightly or wrongly, I felt the calculus of risk had changed. There is the most enormous threat from the combination of this radical extreme movement and the fact that, if they could, they would use nuclear, chemical or biological weapons. You can't take a risk with that happening," The BBC quoted Blair, as saying.
Blair, who led Britain into war in Afghanistan and Iraq, denied that his own policies had fuelled radicalism.
Talking about Chechens, Kashmiris, Palestinians, Iraqis and Afghans, who resisted foreign occupation, he said Western polices were designed to confront radical Islamists because they were "regressive, wicked and backward-looking".
The former Prime Minister, who is now an international envoy to the Middle East for the international Quartet, also called Iran one of the biggest state sponsors of radical Islam, and needs to be prevented by any means from developing a nuclear weapon.
"We need to give a message to Iran that is very clear - that they cannot have nuclear weapons capability, and we will stop them," he added.