9/11 anniversary: We underestimated the number of radical Muslims, says Blair

Mr Blair, who was prime minister when al-Qaida killed thousands in suicide attacks in the US, said the numbers of radical Muslims had been underestimated.

He renewed his stark criticism of Iran, saying regime change in Tehran would make him “significantly more optimistic” about the prospects for the region.

Now a Middle East peace envoy, the former premier also urged the international community to increase pressure on the Syrian regime of Bashar al Assad.

“While the number of extremists is small, we underestimated the numbers who share the narrative of radical Islamism and who believe they are in fundamental conflict with us who do not share it,” he wrote in the Daily Mirror.

“The majority, as the Arab Spring shows, want what we want. But the minority are well organised and very determined and they are not confined to the ranks of suicide bombers.

“It will take a generation of effort at many levels including importantly to support open-minded and tolerant people of all religions to change hearts and minds and make the fanatics irrelevant.”

In an interview with The Times, Mr Blair said: “Regime change in Tehran would immediately make me significantly more optimistic about the whole of the region.”

Apart from the threat of its nuclear programme, Iran had been “one of the main problems” in Iraq and Afghanistan.

The Syrian president had shown he was “not capable of reform”, he said. “His position is untenable. There is no process of change that leaves him intact.”