The shift came late Wednesday when Prime Minister David Cameron's government, aware of the sensitivities created by the Iraq war, said unexpectedly in a parliamentary motion to be debated on Thursday that a separate vote would be required later to authorize military action in response to an attack that left hundreds dead near Damascus last week.
Mr. Cameron bowed to pressure from the opposition Labour Party and to some within his own coalition who want to wait for United Nations weapons inspectors, currently in Syria, to report their findings and for the United Nations Security Council to make one more effort to give a more solid legal backing to military action against Damascus.
The inspectors headed in a six-car convoy on Thursday toward the site of attacks in the eastern outskirts of Damascus for a third day of collecting evidence and samples, news reports said. Ban Ki-moon, the United Nations secretary general, was quoted as telling reporters in Vienna that the inspectors would complete their work on Friday and report to him on Saturday. Their mandate is not to apportion blame for firing chemical weapons but to establish whether they were used.
"A United Nations process must be followed as far as possible to ensure the maximum legitimacy for any such action," the text of the British parliamentary motion said, and the secretary general "should ensure a briefing to the United Nations Security Council immediately upon the completion of the team's initial mission."
"Before any direct British involvement in such action a further vote of the House of Commons will take place," the motion said.
Mr. Cameron, who heads a coalition government, is facing political difficulties from legislators mindful of events in 2003, when assurances from Prime Minister Tony Blair and President George W. Bush that Saddam Hussein had weapons of mass destruction proved to be inaccurate and a false pretext for war.
As Mr. Cameron ran into difficulties, the Syrian government, which has denied accusations by a range of Western and Arab countries that it used chemical weapons in the Aug. 21 attack, moved abruptly to prolong the inspectors' visit. The authorities announced that they had evidence of three previously unreported chemical weapons assaults that they said had been carried out by insurgents and should be investigated by the inspectors.
If the inspectors look into those accusations, they could remain in Syria well past this weekend, beyond their original mandate, as differences swirling around the conflict there move into ever sharper focus.
In an interview published in Paris on Thursday, Ahmad al-Jarba, the president of the fractious Syrian opposition, displayed growing impatience with the pace of Western moves toward a military strike and with the level of support for insurgents seeking the overthrow of President Bashar al-Assad.
Asked in the newspaper Le Parisien what he expected of Western military intervention, he replied: "First of all, a punitive strike against the regime. Then political and military support for the Free Syrian Army. For the Assad regime enjoys total support from Russia, Hezbollah and Iran. We lack everything. Our allies have given us nothing of what we want."
Le Parisien said Mr. Jarba was speaking ahead of a meeting scheduled for Thursday with President François Hollande, whose government was the first in the West to offer formal recognition of the exiled Syrian opposition movement.
In Tehran, meanwhile, President Hassan Rouhani was quoted as saying that Iran, Syria's most powerful regional backer, believed that it was necessary to "apply all efforts to prevent" military action against the authorities in Damascus. "Military action will have a big amount of costs for the region," Iranian state television quoted Mr. Rouhani as telling President Vladimir V. Putin of Russia in a telephone conversation late Wednesday.
Stephen Castle and Steven Erlanger reported from London, and Rick Gladstone from New York. Reporting was contributed by Michael R. Gordon from Washington, Alan Cowell from London, Steven Lee Myers from Moscow, Nick Cumming-Bruce from Geneva and Marlise Simons from The Hague.
