Mr Tony Blair has called on the world's democracies to come together and fight to eradicate "the new evil" of mass terrorism.
And the British Foreign Secretary, Mr Jack Straw, has pledged all possible British help to the US authorities in bringing to justice those responsible for yesterday's unprecedented terrorist attacks on New York and Washington.
Speaking outside Downing Street last night, the Prime Minister vowed to stand "shoulder to shoulder" with the US.
"This is not a battle between the United Sates and terrorism, but between the free and democratic world and terrorism. We, therefore, here in Britain stand shoulder to shoulder with out American friends in this hour of tragedy and we, like them, will not rest until this evil is driven from our world ... I have just chaired an emergency meeting of the British government's civil contingencies committee, and I would like to explain some of the measures that we have agreed to take here. There are a range of precautionary measures.
"We have stepped up security at airports to the highest levels: no flights will take off from the United Kingdom for which we cannot apply the highest standards of security for air crew and passengers. Private flights have been stopped except where specifically authorised. Flight paths into London have been changed so there will be no civil overflights of central London."
As Britain moved to a high state of security alert last night, Queen Elizabeth spoke of her "growing disbelief and total shock" at the unfolding story of devastation and death in the US.
The Conservative leader, Mr William Hague, branded the attacks "a monstrous act of war against the civilised world" and announced the suspension of the count in the Tory leadership election, the result of which was to have been announced this evening. As a mark of respect for those who had died, and by agreement with the two leadership candidates, Mr Hague said the ballots would be counted tomorrow and the result announced by simple statement.
Mr Blair was speaking to delegates at the Trades Union Congress in Brighton as first details emerged about the multiple terrorist attacks.
Explaining that it would be "inappropriate" to proceed with his planned address on public service reform, Mr Blair stayed in Brighton just long enough to speak of the unimaginable "terror and carnage" perpetrated on the US and to offer "the deepest condolences to President Bush and the American people on behalf of the British people" at these "terrible, shocking events".
Before returning to London for a meeting of the cabinet's "Cobra" emergency planning committee, Mr Blair declared: "This mass terrorism is the new evil in the world today.
"It is perpetrated by fanatics who are utterly indifferent to the sanctity of human life, and we, the democracies of this world, are going to have to come together to fight it together and eradicate this evil completely from our world."
Delegates stunned into silence by the breaking news applauded Mr Blair before adjourning their conference for the rest of the day.
Meanwhile, trading at the Stock Exchange was stopped and the City of London evacuated as government buildings throughout the United Kingdom were automatically placed on the highest state of security alert.
As the Prime Minister and the Foreign, Defence and Home Secretaries met security and intelligence chiefs at Downing Street, Queen Elizabeth was kept closely in touch with developments at her Balmoral holiday retreat.
In her message to President Bush, the queen said: "It is with growing disbelief and total shock that I am learning of the terrorist outrages in New York and Washington.
"On behalf of the British people, Mr President, may I express my heartfelt sympathy to the very many bereaved and injured and our admiration for those who are now trying to cope with these unfolding tragedies. Our thoughts and prayers are with you all."