Lawmakers vote Thursday on Britain's response to chemical weapons attacks in Syria -- but approval for military action will require a second vote after the opposition blocked Prime Minister David Cameron's way.
Under growing pressure from MPs who feared Britain was rushing into action, the government was forced to concede 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 last week.
While the political temperature rose, Britain sent six Typhoon fighter jets to its Akrotiri base on Cyprus as a "protective measure", but the defence ministry said the planes will not take part in any direct military action.
The government was said to be outraged by the decision by Ed Miliband, leader of the centre-left opposition Labour Party, to change his stance on Wednesday -- having previously offered the government conditional backing for military action.
The outcome of the vote in the lower House of Commons now hangs in the balance after Labour demanded "compelling evidence" that the Syrian regime had used chemical weapons.
Cameron's government has been forced to dilute the vote to one on merely the principle of military action.
The motion to be debated says that a final vote should only take place after UN inspectors report on the use of chemical weapons in Syria, which Britain and the United States say were unleashed by President Bashar al-Assad.
UN chief Ban Ki-moon said Thursday the investigators would leave Syria by Saturday and report to him immediately.
The Syrian regime strongly denies it was responsible and blames opposition fighters for the attacks.
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.
He said: "I'm clear that this is a very grave decision to take military action that the House of Commons would be making and I didn't think that that decision should be made on an artificial timetable when the House of Commons wouldn't even have seen the evidence today from the UN weapons inspectors.
"I'm determined to learn the lessons of the past, including Iraq, and we can't have the House of Commons being asked to write a blank cheque to the PM for military action."
One government source told The Times newspaper that Cameron had been infuriated by Miliband's change of mind and described him as a "copper-bottomed shit".
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.
Cameron, who recalled parliament from its summer recess for the debate, will try to convince MPs that targeted strikes would punish the Assad regime for its alleged use of chemical weapons and deter any further attacks.
He will also insist that any strikes would not drag Britain into a wider conflict.
Haunted by their experience of the war in Iraq, a growing number of MPs -- including some within Cameron's own centre-right Conservative Party -- are reluctant to back British military involvement.
In 2003, parliament gave 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.
Cameron's spokeswoman said the prime minister was "acutely aware of the deep concerns in the country caused by what happened over Iraq" and was trying to proceed "on a consensual basis".
Deputy Prime Minister Nick Clegg, whose centrist Liberal Democrats are the junior coalition partners of Cameron's Conservatives, sent an email to his MPs Wednesday headlined: "This is not Iraq."
"This is about upholding international and humanitarian law and deterring the use of chemical weapons," Clegg wrote, urging his party to back the motion.
The parliamentary motion says "every effort" should be made to secure approval from the UN Security Council before any military action goes ahead.
But the United States ruled out any chance of securing a UN resolution, blaming "intransigence" from Syria's ally Russia.
