Britain will not take military action against the Syrian regime before UN inspectors report back on evidence of chemical weapons attacks, according to a motion published by the government Wednesday that is set to be put to a parliamentary vote.
Lawmakers are due to vote on Britain's response to the attacks on Thursday but any military action will require a further vote of parliament's lower House of Commons after the UN experts confirm their findings in the coming days.
"Before any direct British involvement in such action a further vote of the House of Commons will take place," the motion said.
The opposition Labour party had threatened to vote against Thursday's motion unless lawmakers were promised a second vote on the basis of the UN evidence -- which had left Prime Minister David Cameron facing possible defeat in his bid for targeted strikes against the Syrian regime.
Along with the United States and France, Britain claims Syrian President Bashar al-Assad's government is responsible for horrific poison gas attacks believed to have left hundreds dead near Damascus a week ago.
The Syrian government strongly denies this and blames opposition fighters for the attacks.
Cameron will on Thursday try to convince lawmakers that targeted strikes would punish the Assad regime for its alleged use of chemical weapons and deter any further attacks.
The parliamentary motion, released by Cameron's office, said "every effort" should be made to secure approval from the United Nations Security Council before any military action goes ahead.
Britain had already sought backing from the council on Wednesday, submitting a draft resolution to its five permanent members calling for action to protect Syrian civilians.
But British ministers concede that a UN resolution supporting military action is extremely unlikely as permanent members Russia and China fiercely oppose a strike against Assad.
Expectations of a Western military assault rose as the United States ruled out any chance of securing a UN resolution, and British Foreign Secretary William Hague said the international community had a responsibility to act even if the Security Council could not agree.
A UN spokesman suggested that the world body's team in Damascus will not report back until at least Friday.
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British lawmakers will be asked on Thursday to back "a strong humanitarian response" that "may, if necessary, require military action that is legal, proportionate and focused on savings lives by preventing and deterring further use of Syria's chemical weapons".
Cameron has recalled lawmakers from their summer break for the marathon debate, in which he is set to propose targeted military strikes to "degrade" the regime's chemical weapons capability.
He will also attempt to persuade lawmakers that any strikes would not drag Britain into the wider conflict.
Some MPs are reluctant to back British military involvement, haunted by their experience of the Iraq war.
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.
"The PM is acutely aware of the deep concerns in the country caused by what happened over Iraq," a spokeswoman for Cameron said.
"That's why we are committed to taking action to deal with this war crime -- but taking action in the right way, proceeding on a consensual basis."
Deputy Prime Minister Nick Clegg, whose Liberal Democrats are the junior coalition partners of Cameron's Conservatives, sent an email to his MPs late Wednesday headlined: "This is not Iraq."
"This is not about regime change," Clegg wrote, urging Lib Dems to back the motion.
"This is about upholding international and humanitarian law and deterring the use of chemical weapons to protect innocent people from being murdered in future by brutal dictators."
A YouGov poll for The Sun newspaper showed that Britons overwhelmingly oppose the use of British missiles against military sites in Syria.
Hundreds of anti-war protesters gathered outside Cameron's Downing Street residence on Wednesday, chanting slogans including "Hands off Syria".
"I'm here today because even though Syrian people are suffering massively at the hands of Assad and his brutal regime, I think a Western intervention would be essentially throwing petrol on the fire," protester Nathan Rodgers told AFP.
"We've seen over the last decade that Western intervention into countries in turmoil only leads to more killing."