The House of Commons will have to hold two votes before it can back "direct" military action in Syria, the government has announced.
MPs, returning early to Westminster on Thursday, will debate a motion on a "strong humanitarian response" to an alleged chemical weapons attack.
A second vote on military action will only happen after the UN has considered reports by its inspectors, it adds.
Labour said David Cameron had "changed his mind" under pressure.
The Motion ...
"This House:
"Deplores the use of chemical weapons in Syria on 21 August 2013 by the Assad regime, which caused hundreds of deaths and thousands of injuries of Syrian civilians;
"Recalls the importance of upholding the worldwide prohibition on the use of chemical weapons under international law;
"Agrees that a strong humanitarian response is required from the international community and that this may, if necessary, require military action that is legal, proportionate and focused on saving lives by preventing and deterring further use of Syria's chemical weapons;
"Notes the failure of the United Nations Security Council over the last two years to take united action in response to the Syrian crisis;
"Notes that the use of chemical weapons is a war crime under customary law and a crime against humanity - and that the principle of humanitarian intervention provides a sound legal basis for taking action;
"Notes the wide international support for such a response, including the statement from the Arab League on 27 August which calls on the international community, represented in the United Nations Security Council, to 'overcome internal disagreements and take action against those who committed this crime, for which the Syrian regime is responsible';
"Believes, in spite of the difficulties at the United Nations, that a United Nations process must be followed as far as possible to ensure the maximum legitimacy for any such action;
"Therefore welcomes the work of the United Nations investigating team currently in Damascus. Whilst noting that the team's mandate is to confirm whether chemical weapons were used and not to apportion blame, agrees that the United Nations Secretary General should ensure a briefing to the United Nations Security Council immediately upon the completion of the team's initial mission;
"Believes that the United Nations Security Council must have the opportunity immediately to consider that briefing and that every effort should be made to secure a Security Council Resolution backing military action before any such action is taken. Before any direct British involvement in such action a further vote of the House of Commons will take place.
"Notes that this motion relates solely to efforts to alleviate humanitarian suffering by deterring use of chemical weapons and does not sanction any action in Syria with wider objectives."
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