The British Prime Minister, Mr Blair, will report to parliament tonight on "a moment of utmost gravity for the world".
The decision to recall the House of Commons was announced shortly after Mr Blair confirmed that British forces - believed to include submarines launching Tomahawk Cruise Missiles - had joined the first assaults on Osama bin Laden and Afghanistan's ruling Taliban regime last night.
Speaking shortly after President Bush's announcement that military action against those believed responsible for the atrocities in the US on September 11th was under way, Mr Blair first acknowledged the "immense burden" and "deep anxiety" shared by members of the armed forces and their families, and by the British people as a whole.
"There is no greater strength for a British Prime Minister and the British nation at a time like this to know that the forces we are calling upon are amongst the best in the world," said a sombre Mr Blair as he vowed the action undertaken by the international coalition led by the US would continue until its objectives were met "in full".
"This is a moment of utmost gravity for the world," he declared. "None of the leaders involved in this action want war. None of our nations want it. We are peaceful people. But we know that sometimes to safeguard peace, we have to fight. Britain has learned that lesson many times in our history. We only do it if the cause is just. This cause is just."
As a group of around 100 peace demonstrators regaled Downing Street with cries of "welfare not warfare" and "we don't want this war", the Archbishop of Canterbury, Dr George Carey, called for prayers for all those caught up in the military strikes on Afghanistan.
Mr Blair, meanwhile, sought to reassure an anxious British public about the possible terrorist response to the US-led offensive.
With an extra 1,500 police officers already on duty around London, and key government and other public buildings on the highest state of security alert, Mr Blair repeated that there was "at present no specific credible threat that we know of," while his government had "in-place tried and tested contingency plans which are the best possible response to any further attempts at terror".
Recalling the hundreds of Britons who died in New York, Mr Blair again described the September 11th attacks as "the worst terrorist outrage against British citizens in our history".
But he continued: "Even if no British citizen had died, we would be right to act. This atrocity was an attack on all of us... We know the al-Qaeda network threatens Europe, including Britain, and indeed any nation throughout the world that does not share their fanatical views. So we have a direct interest in acting in our self-defence to protect British lives."
Mr Blair had the full backing of the Conservative leader, Mr Iain Duncan Smith, who described the allied attack as "justified action against an organisation which has put itself beyond the rule of law". He too offered "all our thoughts and prayers" for Britain's servicemen and women and their families.
"We can take pride in our forces who I believe will bring honour to us all," he said.
However, the Father of the House of Commons, Mr Tam Dalyell - who like Mr Duncan Smith had pressed for the recall of parliament - said many people would be "profoundly uneasy" about the military action, which he believed should have been delayed until the spring if it was to take place at all.
"Heaven knows what the consequences will be in Pakistan, Saudi Arabia, and many other countries without whose goodwill we would have little hope of identifying the perpetrators of the New York crime," he said.