LONDON - Britain's government on Thursday published internal legal advice it had been given which it said showed it was legally entitled to take military action against Syria even if the United Nations Security Council blocked such action.

It also published intelligence material on last week's chemical weapons attack in Syria, saying there was no doubt that such an attack had taken place, that it was "highly likely" that the Syrian government had been behind it, and that there was "some" intelligence to suggest that was the case.

"If action in the Security Council is blocked, the UK would still be permitted under international law to take exceptional measures in order to alleviate the scale of the overwhelming humanitarian catastrophe in Syria," a copy of the British government's legal position read.

In such circumstances, it added that "military intervention to strike specific targets with the aim of deterring and disrupting further such attacks would be necessary and proportionate and therefore legally justifiable." A letter from the chairman of Britain's Joint Intelligence Committee said there were "no plausible alternative scenarios" except that forces loyal to President Bashar al-Assad had perpetrated the chemical attack in the suburbs of Damascus.

"We also have a limited but growing body of intelligence which supports the judgement that the regime was responsible for the attacks and that they were conducted to help clear the Opposition from strategic parts of Damascus," it said.

However, Cameron's plans for an imminent military strike on Syria were in disarray on Thursday after a revolt by lawmakers warning him to heed the "lessons of Iraq".

After imploring the world not to stand idly by over Syria's suspected use of chemical weapons, Cameron was forced into an awkward climbdown on Wednesday when the opposition Labour party and lawmakers in his own party said they wanted more evidence before voting for military action.

The government was due to publish an opinion from one of its top advisers on Thursday about the legality of such a strike, as well as an intelligence dossier expected to back up its assertions that the Syrian government was to blame for an apparent poison gas attack that killed hundreds last week.

Dogging Cameron's steps is the memory of events a decade ago, when Britain helped the United States to invade Iraq after asserting - wrongly, as it later turned out - that dictator Saddam Hussein possessed weapons of mass destruction.

Britain, already embroiled in Afghanistan, was sucked into a second quagmire and lost 179 troops in eight years of war after Iraq descended into savage sectarian conflict.

It was the defining moment of Tony Blair's 1997-2007 premiership, provoking huge protests, divisions within his Labour Party and accusations that his government misled the public by manufacturing the case for war.

"We have got to learn the lessons of Iraq because people remember the mistakes that were made in Iraq and I am not willing to make those mistakes again," said Labour's current leader Ed Miliband.

It was unclear how Cameron's failure to master domestic British politics could affect US and French plans for a swift cruise missile strike against Syria, which denies using chemical weapons against its citizens, or what the impact would be on Cameron's standing in Washington.

President Barack Obama has made the case for a limited military strike on Syria, but some US lawmakers say they have not been properly consulted.

Conservative officials were furious at the delay, accusing Miliband of opportunism.

"Ed Miliband is playing politics when he should be thinking about the national interest and global security," a Conservative source told Reuters. "He keeps changing his position, not out of principle but to achieve political advantage," the source added, saying Cameron wanted to "do the right thing" in the right way.

PUBLIC OPPOSITION

The potent legacy of Iraq is reflected not only in party politics, but in public opinion surveys.

A YouGov poll published on Thursday showed opposition to action hardening, with 51 percent of the British public opposing a missile strike on Syria, and just 22 percent in favor of it. Opponents say Britain has neither the money nor the evidence to justify further military action in the Middle East.

"We do not have a great track record of intervention, there is no appetite for it in the country or really in the House of Commons," said Andrew Bridgen, a lawmaker from Cameron's Conservative party who opposes immediate military action.

Domestically, Cameron's authority looks dented. Part of his problem is that he governs as part of a two-party coalition because his Conservatives lack an absolute majority in parliament, exposing him to such impromptu revolts.

When the prime minister recalled parliament on Tuesday and cut short his own summer break to deal with the Syrian crisis, his rhetoric indicated he was confident of securing parliamentary support for a vote on military action.

But as parliamentarians returned, the tone suddenly changed late on Wednesday: dozens of lawmakers from his own party questioned the evidence of chemical weapons use and warned Cameron he could face defeat unless he toned down his plans.

After hours of negotiations between Cameron's political managers and the opposition, his office agreed that the United Nations Security Council should see findings from chemical weapons inspectors before it responded militarily.

That means that parliament will vote on Thursday on a government motion cautioning President Bashar Assad and authorizing military action in principle only.

It will need to vote again to authorize any direct military action, and Labour is tabling an amendment amid signs it may try to sabotage Thursday's vote. Syria wrote letters to British lawmakers urging them to avoid reckless action.

Cameron, who has the powers of a commander-in-chief, does not technically need parliament's support to order military action. But after tabling a debate and facing such a revolt, it would be hard for him to go against lawmakers' wishes.

"The motion that we're putting forward ... reflects the Prime Minister's recognition of the deep concerns in this country about what happened over Iraq," said Foreign Secretary William Hague.

Britain is to send six RAF Typhoon air-to-air interceptor jets to its Akrotiri base in Cyprus on Thursday, the Ministry of Defence said. Cyprus is just 200 km (120 miles) from the Syrian coast. Britain also has warships in the Mediterranean.

UN Secretary General Ban Ki-moon has sought more time for inspectors to complete their work, a step that could delay any strike as allies would be unlikely to attack with UN weapons inspectors on the ground.

"One of the most important lessons of Iraq is to give the United Nations the proper chance to do its work and I believe if we had tried to make that decision today on military action we wouldn't have been giving the United Nations the proper time to do that work," Labour's Miliband said.

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