British Prime Minister David Cameron faced an uphill struggle on Thursday to secure parliament's approval for military intervention in Syria after the main opposition party said it would vote against the motion.

Opening the debate to lawmakers recalled from their summer recess, Cameron said what was at stake was "one of the most abhorrent uses of chemical weapons in a century".

He insisted that taking action against the Syrian regime's chemical weapons capability was "not about taking sides in the Syrian conflict".

But the outcome of the vote hung in the balance after a party source said the centre-left opposition Labour Party had been having "increasing doubts about the opaque nature of the government's motion".

The motion that lawmakers are being asked to approve "does not mention anything about compelling evidence" that a suspected chemical attack last week outside the Syrian capital was launched by President Bashar al-Assad's forces, the source said.

The Syrian regime strongly denies it was responsible and blames opposition fighters for the attack.

Under growing pressure from MPs who feared Britain 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.

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.

Cameron's government was said to be outraged by the decision of Labour leader Ed Miliband to change his stance on Wednesday -- having previously offered the government conditional backing for military action.

A government source described Miliband as a "copper-bottomed shit", according to The Times.

The 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.

UN chief Ban Ki-moon said Thursday the investigators would leave Syria by Saturday and report to him immediately.

Cameron does not have a clear majority in parliament and his Conservative party is forced to rely on the far smaller Liberal Democrats to rule in a coalition.

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.

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."

Cameron 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.