By Guy Faulconbridge and Andrew Osborn
LONDON (Reuters) - Prime Minister David Cameron's plans for joining a looming 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 as well as lawmakers in his own Conservative party said they wanted more evidence before voting for military action.
On Thursday, Cameron's government published 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 did not approve such action.
It also published intelligence material on the August 21 chemical weapons attack in Syria's civil war, saying there was no doubt that it had occurred and that it was "highly likely" Syrian government forces were responsible. The nerve gas attack killed hundreds of civilians in an embattled suburb of Damascus.
It was unclear how Cameron's failure to master domestic British politics could affect U.S. and French plans for a swift cruise missile strike against Syria - whose government has denied using chemical weapons against its citizens - or what the impact would be on Cameron's standing in Washington.
Aides said he had not spoken to U.S. President Barack Obama since suffering the parliamentary reversal, but that there had been regular contacts at other levels.
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 President Saddam Hussein possessed weapons of mass destruction.
"I am deeply mindful of the lessons of previous conflicts, and in particular the deep concerns in the country caused by what went wrong with the Iraq conflict in 2003," Cameron told parliament on Thursday during a debate on Syria.
"One thing is indisputable: the well of public opinion was well and truly poisoned by the Iraq episode and we need to understand the public skepticism."
Already embroiled in Afghanistan, Britain was then sucked into a second quagmire in Iraq, losing 179 soldiers in eight years of militant attacks and sectarian conflict following the 2003 U.S.-British invasion that toppled Saddam Hussein.
POISONOUS IRAQ LEGACY
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.
Obama has set out the case for a limited military strike on Syria, but some U.S. lawmakers say they have not been properly consulted.
British Conservative officials were furious at the delay, accusing Labour leader Ed 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."
A senior Conservative source added: "A lot of the arguments over this could give succor to the (Syrian) regime".
Miliband replied: "We have got to learn the lessons of Iraq because people remember the mistakes that were made ... I am not willing to make those mistakes again." He advised lawmakers not to rush to judgment "on a political timetable set elsewhere".
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.
After hours of negotiations between Cameron's political managers and the opposition, his office agreed that the U.N. Security Council should see findings from chemical weapons inspectors before it responded militarily and that parliament should hold two votes on military action.
That meant that parliament would vote later on Thursday on a government motion cautioning Syrian President Bashar al-Assad and authorizing military action in principle only.
It will need to vote again to authorize any direct military action. Labour said it would vote against the government on Thursday, while Syria wrote letters to British lawmakers urging them to avoid reckless action.
A YouGov poll published on Thursday showed opposition to a Syria action hardening, with 51 percent of the British public opposing a missile strike, 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.
(Additional reporting by William James, Belinda Goldsmith and Costas Pitas in London and Michele Kambas in Cyprus; Editing by Mark Heinrich)