British Prime Minister David Cameron made his case on Thursday for military action against Syria in the face of strong resistance from the opposition and public scepticism.
Cameron told lawmakers recalled from their summer recess that Britain could not stand idle in the face of "one of the most abhorrent uses of chemical weapons in a century".
He said that while he was convinced the Syrian regime was behind the gas attack that is said to have killed hundreds near Damascus last week, there was no "100-percent certainty" and lawmakers had to "make a judgment".
Cameron insisted that launching strikes to degrade the regime's ability to use chemical weapons was "not about taking sides in the Syrian conflict".
But the outcome of the vote hung in the balance after the centre-left opposition Labour Party indicated it would vote against the government's motion, which asks MPs to approve military action in principle -- although a second vote on another day will be required before attacks can be launched.
Under growing pressure from MPs who feared London was rushing into action, the government was forced to agree late Wednesday that Britain would not take part in any military strikes before United Nations inspectors report back on the gas attacks believed to have killed hundreds near Damascus on August 21.
Labour leader Ed Miliband told a packed House of Commons chamber: "The weapons inspectors are in the midst of their work and will be reporting in the coming days -- that is why today could not have been the day when the house was asked to decide on military action."
The Syrian government has denied it was responsible for the suspected chemical attack and has blamed opposition forces for launching it.
US President Barack Obama has said he is yet to decide on a plan of action for Syria.
UN chief Ban Ki-moon said the investigators would leave Syria by Saturday and report to him immediately.
While the political temperature rose, Britain dispatched six Typhoon fighter jets to its Akrotiri base on Cyprus as a "protective measure", although the defence ministry said the planes will not take part in any direct military action.
Signup to our newsletter and follow us on Facebook and Twitter!
A senior Labour source said the government motion does not "mention anything about compelling evidence" that the chemical attack was launched by Assad's forces, and therefore its MPs were likely to vote against the government.
Cameron's centre-right Conservative Party does not have a majority in parliament and they govern in coalition with the far smaller centrist Liberal Democrats.
The government was said to be outraged by Miliband's decision to change his stance on Wednesday -- having previously offered the government conditional backing.
A government source described Miliband as a "copper-bottomed shit", according to The Times newspaper.
With British lawmakers now facing the prospect of having to vote for a second time on a different day -- possibly early next week -- it raises the possibility that the United States will go it alone with missile strikes, without involvement from Britain, its main military ally.
Muddying the waters, the government also said it had received legal advice that under international law, Britain could still launch military action even without a mandate from the UN Security Council.
Miliband is pushing ahead with his own amendment that calls for a greater UN role before any military action is authorised, and has not said whether the party will support the government if that is rejected.
Haunted by their experience of the war in Iraq, a growing number of MPs -- including some Conservatives -- are reluctant to back British military involvement.
In 2003, parliament gave Labour's then prime minister Tony Blair a mandate to join the US-led offensive in Iraq on the basis of allegations that dictator Saddam Hussein had weapons of mass destruction.
The weapons never materialised and Britain became embroiled in the war for years.
Public opinion in Britain is against missile strikes. A YouGov poll for The Times found that support for firing missiles against military sites in Syria had dropped on Wednesday to 22 percent, from 25 percent on Tuesday, while opposition grew from 50 percent to 51 percent.