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David Cameron must today face the ire of his political colleagues as well as his opponents, as he prepares for a crucial House of Commons debate and vote on whether Britain will go to war against the Islamic State group in Syria.

The British prime minister branded citizens and parliamentarians opposed to a military air campaign "terrorist sympathisers" with a rhetorical flourish reminiscent of George W Bush's binary "with-us-or-against-us" attitude towards warfare.

The barb, aimed at opposition leader Jeremy Corbyn and his supporters, appeared to have backfired in remarkable fashion, with analysts suggesting he was "stamping his feet" like a child wanting to get his way.

It was "a contemptible and desperate slur which demeans his office", Corbyn's Labour party told The Guardian.

Twitter users last night were having a field day, with many claiming hypocrisy on Cameron's part, pointing to the prime minister's recent endorsement of some regimes with spectacularly poor human rights records, including Saudi Arabia, Egypt and China.

"Three and an arms dealer walk into a bar... Where they were entertained as guests of the British government," posted .

"If wanting peace and having concern for innocent people makes me a , then I am guilty. Aggression is not always the answer," posted .

Relying on politician's consciences

Despite the ruling Conservative Party's majority in parliament, it was not clear that a resolution calling for Britain's Royal Air Force to join the US-led anti-IS airstrike coalition would receive enough votes to pass - until Labour leader Corbyn refused to enforce a "whipped vote", allowing opposition politicians "to vote with their conscience".

"I hope every MP will recognise tomorrow there is no hiding place on whipping or anything else, you have got to make up your own mind," Corbyn told Sky News on Tuesday night. 

"It seems to me that we are stepping into something that is potentially very dangerous and rather unknown."

It is thought the resolution will pass with around 360 votes calling for an aerial campaign, and around 170 rejecting the proposed bombing.

	Thousands gathered to protest against the proposed bombing [Getty]

But Cameron and his closest circle faced rebellion from within his own party on Tuesday night, as leading Conservatives said the prime minister had failed to make the case for bombing.

The Conservative-led seven-member Foreign Affairs Committee voted by four to three to say that Cameron "had not adequately addressed concerns".

A central plank in the prime minister's case is his claim that there are some 70,000 "moderate" fighters in Syria that would be bolstered by Britain's aerial support.

But that figure has come under intense scrutiny by lawmakers and a public still wary of the infamous "45-minute" claim of then-Prime Minister Tony Blair, which largely led Britain into its disastrous invasion of Iraq in 2003.

Lieutenant General Gordon Messenger, the deputy chief of the UK defence staff, told politicians on Tuesday that the secrecy surrounding national security operations meant he could not confirm the composition of the 70,000 fighters - or, indeed, whether that included members of the Islamic State group itself.

Speaking to Sky News, Corbyn again decried the prime minister's claims. "[Cameron is] claiming there is an army of 75,000 people - whose existence is questionable, shall we say, and whose membership is certainly more interested either in fighting Assad or in doing deals with other jihadist forces."

John Baron MP is a Conservative member of the foreign affairs committee. On Tuesday, he announced he would also be voting against the prime minister's plan of attack.

"Though the government has precipitated another rush into military action, it has not yet constructed a realistic long-term strategy to destroy [IS]," he in The Guardian.

"All the previous military interventions [Iraq, Libya, Afghanistan] were ill fated from the outset, because of the lack of a comprehensive plan coupled with a dearth of local knowledge and history."

Protest

Bombing Syria is also far from popular among the public, with Wednesday's Times front page declaring that fewer than half of British voters support Cameron's military campaign.

Thousands of anti-war protesters turned up outside parliament on Tuesday night, shouting "don't bomb Syria". Police dragged protesters off the road after traffic was blocked by an impromptu sit-in demonstration.

"Do not go and bomb a country where you make it even worse than it was before," Lindsay German, of the Stop The War Coalition, told the crowd.

A group of 110 MPs from across Britain's political spectrum had last night signed a cross-party amendment to the resolution. The amendment, if passed, would effectively kill the legislation and cancel the proposed military action.  

"We are seeing an arrogance from David Cameron who believes his ill-conceived plans to add more planes to the 10 countries already bombing Syria will make a difference," said Alec Salmond MP on Tuesday night.

Jeremy Corbyn, who swept to the Labour leadership this summer on a wave of leftist revivalism, has come under fire both for not forcing opposition MPs to follow his lead in voting against the government, and for reportedly encouraging the party's representatives to attend a by the journalist Patrick Cockburn.

The Stop The War coalition's Andrew Murray said Corbyn's decision had "cleared the way" for airstrikes.

A noted rebel who voted with his conscience rather than his party's leadership during his many years as a Labour backbencher, Corbyn would likely have been widely pilloried if he had attempted to enforce "discipline".

But the decision to recommend Cockburn as an expert witness on Syria's troubles has left Corbyn vulnerable to incoming flak. Cockburn, an award-winning journalist who has spent decades reporting from front lines in Iraq and across the Middle East, for the Financial Times, The Independent and many more, has been in recent years of being an apologist for Syrian President Bashar al-Assad.

The debate gets underway in the House of Commons at 11:30am, and a full day of speeches has been scheduled. The vote will take place some time around 10pm.

The bombers will undoubtedly already be standing ready, fully fuelled - and fully armed.

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