MPs will be given a vote over potential British military action against the Bashar al-Assad regime when parliament is recalled later this week, David Cameron has confirmed.
The prime minister promised "a clear Govt motion & vote on UK response to chemical weapons attacks" in a tweet setting up a crunch vote on Thursday.
It remains unclear whether the vote will be technically binding, but pressing ahead with an armed intervention in Syria would be close to politically impossible were Cameron to lose the division.
The prime minister said that any action would be "legal", "proportionate" and "specific".
In a statement designed to alleviate fears that the west may be planning a programme of regime change, he insisted any intervention would be launched only to limit the use of chemical weapons.
"The question we have to ask ourselves is if there is no action following this big use of chemical weapons is it going to be more likely in future that more and more regimes will use chemical weapons?" he said.
"That this regime will use them again and again on a larger scale and we'll see more death and more suffering?
"The question we need to ask is whether acting or not acting will make the use of chemical weapons more prevalent."
Military contingency plans are already being drawn up by the Ministry of Defence despite question-marks remaining over the legality of any military action, the binding nature of any Commons vote and the government's intention to publish the evidence justifying an armed intervention.
The rush to secure the Commons' backing reinforces the impression that Britain is set to ignore the UN team's evidence on chemical weapons in Syria.
"From our own evidence, it is clear that chemical weapons were used and that they were used by the Assad regime," the prime minister's spokesman said this morning.
He suggested the evidence being collected from the UN inspection team could be compromised because of the Syrian government's delay in allowing them access.
"There is a concern the fact the regime did not allow that team in on day one suggests their evidence could have been tampered with," the spokesman added.
Meanwhile, the Syrian Coalition alleged this afternoon that regime forces hit Aroum Al Kubra with internationally prohibited phosphorus bombs & napalm, killing ten.
Urgent military action could go ahead without evidence from the UN as a result. It is far from clear whether the Commons would approve an attack on Assad's military assets before it takes place.
"When we see a crisis of this sort, the use of chemical weapons against the regime's own people, there needs to be a response and that response needs to come from the international community," the PM's spokesman explained.
"We are currently in discussion with our international partners about what that response should be."
News of the vote will be welcomed by Labour, which had demanded that MPs be allowed a say on any strike against the Syrian government. Many MPs had backed an early day motion by Graham Allen demanding a recall of parliament, with prominent backbenchers like Douglas Carswell, David Davis, Graham Stuart and Phillip Davies lending their support.
"We have a good record on consulting parliament - having a vote in parliament if we decide to take any military action and we're conscious of the views of parliament on these matters and the need to be consulted," foreign secretary William Hague said before the recall announcement was made.
"We're very conscious of that, but our decisions on that will depend on the timing and nature of what we propose to do."
Britain is part of a broader diplomatic effort to secure consensus from the international community over action against Assad. David Cameron has spoken to the leaders of the US, Russia, Germany, France, and Canada in recent days as part of moves to establish a 'coalition of the willing'.
The prime minister, who returned to No 10 from his break in Cornwall this morning, will to hold a national security meeting tomorrow, as UK army chiefs meet their US and international counterparts to discuss action.
The UK and US will need to establish how they would pursue military action without the consent of the UN security council, where Russia and China will block any attempt to intervene in the conflict.
"Is it possible to act on chemical weapons, is it possible to respond to chemical weapons without complete unity on the UN security council?" Hague asked.
"I would argue, yes it is. Otherwise, of course, it might be impossible to respond to such outrages, such crimes and I don't think that is an acceptable situation.
"It is possible to take action based on great humanitarian need and humanitarian distress, it’s possible to do that under many different scenarios."
Shadow of Iraq
The debate over parliamentary consent and security council authorisation reflect the extent to which the shadow of the Iraq war still looms large over British defence decision-making.
That fact was not lost on former prime minister Tony Blair, who urged those who opposed Iraq to accept the need for intervention in Syria.
"I know as one of the architects of policy after 9/11 the controversy, anguish and cost of the decisions taken," he said.
"I understand why, now, the pendulum has swung so heavily the other way. But it is not necessary to revert to that policy to make a difference.
"After the long and painful campaigns in Iraq and Afghanistan, I understand every impulse to stay clear of the turmoil, to watch but not to intervene, to ratchet up language but not to engage in the hard, even harsh business of changing reality on the ground.
"But we have collectively to understand the consequences of wringing our hands instead of putting them to work."
Plaid Cymru called on the government to avoid aligning itself with supporters of military action, warning that recent interventions in Iraq, Afghanistan and elsewhere had not been successes, however.
"The commitment of troops without an end plan - an extraordinary high proportion of whom come from Wales, costs a high price financially in addition to the obvious costs to families and communities from loss of life or physical or mental injury," party leader Leanne Wood said.
Behind closed doors
In a gripping insight into the real politick behind the scenes of the Syria crisis, it emerged that Saudi Arabia tried to offer Russia a game-changing oil agreement in return for Vladimir Putin's pledge to drop his support for the Assad regime in Syria.
Saudi Arabia reportedly offered an OPEC-Russian alliance, uniting 45% of global oil output – a move which would fundamentally alter the strategic environment in energy and have a significant impact on the global economy.
It is also understood to have offered a guarantee that no Chechen terrorists would strike the Winter Olympics in Sochi, Russia.
As part of the deal the Saudis would probably put pressure on Qatar to cut output of liquefied natural gas, thereby boosting the price, and perhaps even restrict its own supply.
Any such move would be a massive boost to Russia, which relies on high gas prices to stay out of recession.
However, the fact that details of the deal emerged in the Russian press – before being further fleshed out in the anti-Saudi As-Safir Lebanese newspaper - suggests Moscow rejected the offer.
Cameron spoke to Putin last night and told him there was "little doubt" the Assad regime was responsible for last week's chemical attack.
That claim is rejected by Russia, which says there is no evidence of Syria government involvement or even that the attack ever took place.
Some say the use of chemical weapons, which have long been considered a red-line for US involvement, was a counter-productive move by the regime.
Assad has had the upper hand and the momentum in the ground war for several months and it would have been a strategic misstep to use chemical weapons when doing so would force US president Barack Obama's hand.
But the horrifying videos of children and elderly people suffering from a chemical attack which emerged online last week would have been extremely difficult for rebel forces to fabricate and analysts do not believe they have the ability to fire chemical weapons themselves.
Drums of war
"What we saw in Syria last week should shock the conscience of the world. It defies any code of morality," US secretary of state John Kerry said yesterday.
"Make no mistake, President Obama believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people."
Chemical weapon use is very difficult to prove after 48 hours, so suspicions were raised when Syrian authorities only allowed UN weapons inspectors into the area to test yesterday, several days after the attack
"Instead it attacked the area further, shelling it and systematically destroying evidence. That is not the behaviour of a government that has nothing to hide. The regime's belated decision to allow access is too late and is too late to be credible," Kerry added.
The UN inspectors were fired on by a sniper as they made their way into the area yesterday, but they proceeded to one of five locations around Damascus where the weapon is thought to have been released
They interviewed witnesses, survivors and doctors and collected samples from two hospitals. They are also set to visit areas in the eastern suburb of Ghouta.
Assad warned yesterday that military action would repeat the mistakes of Iraq.
"If someone is dreaming of making Syria a puppet of the West, then this will not happen," he said.
But Washington has recently consolidated its military presence in the eastern Mediterranean. That chimes with the most likely form of military intervention – using sea-launched rockets to target Syria military installations.
The UN estimates that 100,000 people have been killed since the start of the Syrian conflict two years ago, with 1.7 million refugees.