MPs will be recalled to Parliament for a debate and vote on Syria on Thursday, David Cameron announced today.
The UK Prime Minister said there would be a clear Government motion laid before MPs on the UK's response to the chemical weapons attack on the outskirts of Damascus that left hundreds dead.
Mr Cameron returned to Downing Street this morning after cutting short his summer holiday to meet Cabinet colleagues and continue talks with international leaders to find a "proportionate" response which will "deter" the Syrian state from using toxic agents.
He wrote on Twitter: "Speaker agrees my request to recall Parliament on Thurs. There'll be a clear Govt motion & vote on UK response to chemical weapons attacks."
The recall comes after Downing Street confirmed that Britain's armed forces are drawing up contingency plans for military action.
Syrian President Bashar Assad's regime has denied "utterly and completely" that it was behind the atrocity and has warned that military action would be a violation of international law and doomed to fail.
No 10 indicated that an agreement on the next steps could be taken before the results of a report by UN weapons inspectors into the attack is produced but insisted that any response would be legal.
"No decision has yet been taken," a Downing Street spokesman said. "We are continuing to discuss with our international partners what the right response should be, but, as part of this, we are making contingency plans for the armed forces."
Cabinet ministers, including Defence Secretary Philip Hammond, have arrived at No 10 for talks ahead of a national security council (NSC) tomorrow which will discuss possible UK involvement in using force.
Syrian foreign minister Walid Muallem today denied "utterly and completely" that state forces had been behind the attack and warned that the nation would defend itself by "all means available".
At a press conference in Damascus, he said: "They said that the Syrian forces, the Syrian army are the ones who did this attack.
"I deny it utterly and completely.
"There is no country in the world who would use an ultimate destruction weapon against his own people."
Mr Cameron told world leaders, including US President Barack Obama and Russian President Vladimir Putin, over recent days that the international community "needs to respond" to the "completely and utterly abhorrent and unacceptable" attack, Downing Street said.
A spokesman added: "In terms of end-game, this is about looking at how we deter the use of chemical weapons because this is something that is completely abhorrent and against all international law.
"This is about deterring the use of chemical weapons."
The Government is looking at a "range of evidence" about the atrocity that international medical humanitarian organisation Medecins Sans Frontieres (Doctors Without Borders) (MSF) said killed 355 people.
"There is evidence that will come back from the UN so we will look at that but there is still a parallel process going on which is looking at the evidence we already have, the evidence that the Americans and our other international partners have," the Downing Street spokesman said.
Asked if a decision on action was not necessarily dependent on the results from UN weapons inspectors, he said: "Yes. What I am saying here is there is a process which is going on and we are in discussions with our international partners looking at the evidence that is available."
Moscow - a key regime ally which supplies arms to Syria - has backed Syrian claims that video footage of victims could be opposition propaganda.
Mr Cameron clashed with President Putin over Russia's continued insistence that there is "no evidence" of a chemical attack.
US Secretary of State John Kerry has said the attack was a "moral obscenity" which "should shock the conscience of the world" and promised action to hold the regime accountable.
Peers will also be recalled to Parliament, according to the Labour whips' office.
Conservative MP John Baron, who staged a Commons debate on parliamentary approval for action in Syria before the summer recess, said he "very much welcomed" the decision.
He said: "I think Parliament is understandably sceptical, many in Parliament are sceptical (about intervention).
"I don't think there is any doubt chemical weapons have been used, but there is doubt whether Assad used them.
"We also need to debate the implications of armed intervention without a United Nations resolution."
Asked what the implications might be if action was taken against the consent of Parliament, Mr Baron, MP for Basildon and Billericay, said: "There is no point putting a motion to the vote if we are not going to abide by it."
Conservative Richard Ottaway, chairman of the Commons Foreign Affairs Committee, said international law had changed since intervention in Kosovo, with the UN introducing humanitarian doctrines that meant action in the face of a veto would have no legal authority.
"That doctrine only supports intervention with the backing of a UN resolution, so if China and Russia veto any resolution, then clearly an intervention would not have the legal authority," he told BBC Radio 4's The World At One.
"But nonetheless I think that the nature of what's going on in Syria now is so fundamental that it would be legitimate to carry out an action."
He added: "Unlawful is probably a slightly exaggerated phrase. I think there is no legal support for an intervention of this nature rather than describing it as illegal."
Former foreign secretary Jack Straw said MPs had been made more sceptical by the decision to invade Iraq, which occurred during his time in office.
He told the BBC that Mr Cameron "will need to make the case for the kind of proportionate military response that I understand he and our American, French and Turkish colleagues are now considering".
The Labour veteran said: "There is an instinct in the British House of Commons to support a British prime minister, of whatever party, where he or she is recommending military action.
"So that will be part of the instinct, but also there is no doubt that the experience of Iraq has raised the bar of scepticism by the British House of Commons on behalf of the British people about whether military action is justified."
He said Parliament had "not turned pacifist", as the approval for military action in Libya demonstrated, but "it has become more questioning".
Robert Halfon, Conservative MP for Harlow, said: "Parliament is now going to debate the issue; I very much hope politicians of all parties will vote to stop mass genocide and the use of chemical weapons.
"We don't know what the motion is going to be yet."
North West Leicestershire MP Andrew Bridgen, who sent a letter signed by 81 Conservatives to Mr Cameron demanding a vote earlier this year, said: "The House is going to seek assurance on the grounds for action, that there is compelling evidence it is the Assad regime that launched the chemical attacks - that will need to be proved and explained.
"We will need the aims of any action and limits and scope of action, and information on who else will be involved."
Foreign Secretary William Hague has cancelled plans to meet Swedish foreign minister Carl Bildt in Stockholm tomorrow in light of developments in Syria and the decision to recall Parliament, the Foreign Office confirmed.