Prime Minister Tony Blair vowed yesterday that Britain would join America in hunting down those responsible for the "hideous and foul" terror strikes on the US.

He told a hushed emergency session of the Commons that the UK had "not just an interest but an obligation to bring those responsible to account".

Mr Blair said the murder of "at least 100 British citizens, maybe many more" in the attacks should be treated as if they had been killed "in the heart of Britain itself".

Flanked by Foreign Secretary Jack Straw and Defence Secretary Geoff Hoon, Mr Blair stressed: "We are talking here about a tragedy of epoch-making proportions."

New Tory leader Iain Duncan Smith immediately offered the Government his party's backing, saying Mr Blair had "our full support for his immediate pledge to stand shoulder to shoulder with our strongest friends and allies in the US".

He added: "Together we must make sure that the perpetrators are hunted down and brought to justice."

Mr Blair told a packed and sombre Commons in his 14-minute statement that "appropriate action" would be taken once those responsible and those who had aided them had been identified.

And he warned states harbouring terrorists "either to cease their protection of our enemies or be treated as an enemy themselves".

The Prime Minister repeated his promise to back America: "We the British are a people that stand by our friends in time of need, trial and tragedy and we do so without hesitation now."

The Commons and the House of Lords, also hearing an emergency statement, joined the rest of the nation and Europe in observing the three-minute silence at 11am.

Mr Blair took pains to stress that if so-called Islamic fundamentalists were behind the attacks, "we know they do not speak or act for the vast majority of decent, law-abiding Muslims throughout the world".

But in urging wider international action against terror groups, Mr Blair issued a chilling warning.

He told MPs: "We know that they would, if they could, go further and use chemical, biological or even nuclear weapons of mass destruction."

And he said the civilised world held its own belief as strongly as those of the "fanatic" terrorists, adding: "Now is the time to show it."

He warned that action against those responsible "will be determined, it will take time, it will continue over time until this menace is properly dealt with and its machinery of terror destroyed".

Veteran Labour MP Tam Dalyell, Father of the House as the longest-serving MP, urged Britain not to kill innocent civilians in revenge attacks.

"If we do so, we simply recruit more terrorists," said Mr Dalyell.

Liberal Democrat leader Charles Kennedy and Ulster Unionist leader David Trimble joined in condemnation of the attacks. Mr Kennedy reflected the grim mood of MPs as he told them: "I think the angel of death is very much upon us today.