British military responses to the terrorist attacks on America were discussed by the Cabinet after Tony Blair briefed his ministers on the Government's reaction to the crisis.
Admiral Sir Michael Boyce, the chief of the defence staff, visited Downing Street to brief Mr Blair and other senior ministers on the latest security and diplomatic situation.
Few details of the discussions were released but Mr Blair's official spokesman reiterated that Britain would stand by the United States.
He emphasised the importance of Wednesday's decision to invoke clause five of the Nato treaty, which classifies an attack on one member as an attack on all. But he insisted it was not a "blank cheque" for any American action.
Asked whether Britain was at war with the terrorists, Mr Blair's spokesman said: "The Cabinet discussed some of the military issues that follow from this."
Downing Street declined to discuss possible military options. The spokesman said: "It will be for the US to decide on the appropriate action against those responsible. The questions that follow from that for us can obviously be discussed at the appropriate time.
"What is very, very important from the last 24 hours is that we have seen the international community coming together. The European Union, Nato, the United Nations Security Council resolution and the international community stands full square behind the US and shares its horror at what has happened."
Meanwhile, senior MPs warned the British and American governments not to rush to military action, warning of the danger of exacerbating tensions in the Middle East and provoking further terrorist attacks.
Lord Ashdown warned against the use of the word "war" to describe the situation. He said: "If we mishandle this, it is not impossible to see this as the beginning of events which may lead to a genuine war between nations."
The senior Labour backbencher Tam Dalyell warned against a "rush to judgement". He said: "There's a great difference between bringing people to justice ... and massive retaliation. Massive retaliation is probably what people like bin Laden want, innocent people killed. If that happens the genuine sympathy for the Americans from many Arab countries will simply evaporate. We had better be jolly careful about launching any military offensive against Afghanistan."
Earlier Jack Straw, the Foreign Secretary, said Nato and the United Nations Security Council were both resolved to take whatever action was necessary to hold to account the terrorists, and those who sheltered them. He said: "This was absolutely calculated, a calculated attack on democracy, on civilisation."
He promised full co- operation between British and American intelligence services and called for "a determined response, a response that is based on judgements, and of course which is based on evidence".
Mr Blair and senior ministers were briefed early yesterday at a meeting of the Government civil contingencies committee, before the Prime Minister briefed ministers at a "very sombre" emergency cabinet meeting. Mr Blair told the 40-minute meeting the tragedy would have "great ramifications for communities across Britain".
His spokesman said: "He made the point that if this was an event which had happened in the UK with such a large amount of individuals feared dead you could only imagine the shock.
"The Prime Minister said this would be a tragedy which would touch every community here as the numbers of dead and the names of those dead became known to us."
The cabinet meeting, which was called before an emergency statement in Parliament today, covered the immediate response to the crisis and the "slower time" implications for security and the fight against terrorism.
The spokesman said: "As the Prime Minister has said, we have to be vigilant and we have to put in place appropriate cautionary measures. Clearly, if you are dealing with a phenomenon of people who have no compunction at blowing themselves up, you are dealing with a wholly new threat. We are all at risk from the threat of global terrorism."
Security at airports is being kept at the highest state of alert and the no-fly zone over central London will remain in force, the Cabinet was told. But the ban on private flights has been lifted and London City Airport will be allowed to reopen, probably today. Security measures will be kept under review.
The Chancellor, Gordon Brown, told the Cabinet that Tuesday's events had been an attack on the world's financial infrastructure. Mr Blair's spokesman said: "He said this had been an attempt to bring a halt to the global economic system but that it had not been allowed to happen. Markets in Europe and London had remained open."
Tessa Jowell, the Secretary of State for Culture, Media and Sport, has been given the role of helping with funeral arrangements for the hundreds of Britons feared killed.
Ministers were at pains to distance British Muslim communities from the activities of Islamic militants. Pressed on measures to remove extremists, Mr Blair's spokesman hinted at moves to tighten controls on militants currently living in Britain.
He warned far-right groups not to exploit the attacks on America, saying it was important to stress that extremists should not use the atrocities as an excuse to perpetrate their own racial discontent.Reuse content