Prime Minister David Cameron bowed to mounting pressure and recalled MPs in a lunchtime tweet.
MPs would be given a vote on a "clear government motion" on the response to chemical weapons attacks in Syria, he announced.
Mr Cameron claimed in a TV interview that "no decision has yet been taken" on military action.
He refused to say what would happen if MPs were to vote against his motion.
But Mr Cameron's aides sought in advance to justify military action by claiming that Britain and the US already possessed evidence that the Syrian government had used chemical weapons.
Stop the War campaigners announced a protest outside Downing Street at 5pm today, warning that "any attack on Syria can only inflame an already disastrous civil war."
Left Labour MP Jeremy Corbyn predicted that military action would have "incalculable consequences" for Syria and the whole Middle East.
"Goodness knows how many more deaths there will be as a result," he declared.
Warmonger Tony Blair stoked up the flames of conflict today by demanding decisive military action.
Journalists goaded Mr Cameron's spokesman by asking if there would be a dossier of "evidence" along the lines of the "dodgy dossier" used by former PM Blair to justify the Iraq war.
The spokesman was cagey, insisting only that the British government was "looking at a range of evidence" on the use of chemical weapons.
All the evidence "leads us to believe that this is the work of the Assad regime," he claimed.
Asked whether Britain would wait for the UN inspectors' report, the Downing Street spokesman replied: "All I am going to say is that we are continuing to discuss with our international partners on the next steps."
Britain's armed forces were "making contingency plans," he said.
And he insisted: "Any use of chemical weapons is completely and utterly abhorrent and unacceptable. And the international community needs to respond to that."
Mr Cameron will preside over a meeting of the National Security Council today, attended by military chiefs.
Left Labour MP Katy Clark joined 60 MPs who pressed the government with a cross-party motion demanding that Parliament "should hold a full debate before any British commitment to military action in Syria."
Ms Clark said: "It is an explosive situation and we should proceed with great caution."
Several Tory and Lib Dem chairs of select committees signed the urgent call for a debate, which was drawn up over the holiday weekend by Graham Allen, Labour chair of the constitutional reform committee.
Tory chairman of the foreign affairs committee Richard Ottoway voiced cautious support for a missile strike against Syria.
But he added: "I think in order to convince a fairly sceptical Parliament of the need for this course of action, they are going to have to be pretty forthcoming with the evidence."
Tory MP John Baron warned that armed intervention could make the situation worse and questions must be answered in tomorrow's debate.
"Verification is important. There have been claims and counter-claims about the use of chemical weapons on both sides," he said.
Plaid Cymru leader Leanne Wood opposed military action, warning against escalating "an already toxic and dangerous situation."
Labour leader Ed Miliband and shadow foreign secretary Douglas Alexander appealed to Mr Cameron to explain clearly to Parliament the objectives, the legal basis and the "anticipated effect" of any military action.
Mr Alexander said he was "not prepared to write the government a blank cheque" on military action.