We know they would, if they could, go further and use chemical or biological or even nuclear weapons of mass destruction

Tony Blair

Mr Blair said other recent losses of UK lives in the Lockerbie bombing, the Falklands and the Gulf were not on the same scale as the attack on America, which had left at least 100 Britons dead.

Mr Blair also spoke to US President George Bush by telephone during the day.

A Downing Street spokesman said simply: "Their conversation was part of a regular series of conversations with President Bush and other world leaders."

Grave situation

Later Europe Minister Peter Hain warned of the seriousness of the current state of affairs.

"It is important that everybody understands the gravity of the situation.

"Those guided missile aeroplanes could have be pointed at Buckingham Palace or Whitehall or Canary Wharf and they still could be in the future unless we crack down on this kind of really serious, harmful and brutal terrorism," he said.

Speaking in Parliament Mr Blair pledged to bring to account the perpetrators of the "hideous" atrocities, who were the enemies of the civilised world.

"Those that harbour and help them have a choice - either to cease their protection of our enemies or be treated as an enemy themselves."

Nuclear fear

Mr Blair said he feared the terrorists could escalate their activities to use weapons of mass destruction.

Blair: Terrorists could go further still

"We know that these groups are fanatics, capable of killing without discrimination. The limits on the numbers they kill are not governed by morality. The limits are only practical or technical.

"We know they would, if they could, go further and use chemical or biological or even nuclear weapons."

He said that the attack in the US heightened the need to crack down on the trade in the technology to produce and use such weaponry.

"It is time this trade was exposed, disrupted and stamped out. We have been warned by the events of the 11th of September. We should act on the warning."

Mr Blair's spokesman later said the government did not believe terrorist groups yet had nuclear, chemical or biological capability, but that they had the desire to get them.

Serious intent

The prime minister told MPs, some of whom had been specially flown back from America, why he had decided to recall Parliament.

"These were attacks on the basic democratic values in which we all believe so passionately and on the civilised world."

Like other party leaders, Mr Blair again stressed that if Islamic fundamentalists were the perpetrators, it did not mean decent, law-abiding Arabs or Muslims were in any way to blame.

Both the Commons and the Lords observed the three-minute silence which took place across 43 countries as a mark of respect at 1100 BST.

'Act of war'

Iain Duncan Smith responded to the prime minister's emergency statement in his Commons debut as Conservative leader.

Duncan Smith: The horror is virtually incomprehensible

He said democracy must always triumph over evil: "It is the responsibility of civilised countries everywhere to do whatever is necessary to prevent such atrocities ever happening again."

The new Tory leader said: "We now have the opportunity to support the United States in defence of freedom and democracy as they have done for us in the past."

Liberal Democrat leader Charles Kennedy expressed his party's horror at the "breathtaking nature of Tuesday's savagery".

Limits of action

But doubts about how far the British government should support US action came from some Labour backbenchers.

Father of the House Labour MP Tam Dalyell asked the government to promise not to "inflict terror on innocent people".

Such action would only breed more terrorism, he argued.

Friday is a Europe-wide day of mourning to remember the victims of the terror attacks.

To mark it the Queen, accompanied by the Duke of Edinburgh and Prince of Wales, joined Prime Minister Tony Blair and the American ambassador in London, William Farish, at a special service of remembrance at St Paul's Cathedral in London.