Labour MPs who support military action in Syria will have to defy party members after they voted against a bombing campaign without UN approval.
Delegates at the Labour Party conference in Brighton backed an emergency motion that set out strict conditions before the party will back any move by David Cameron to bomb Isis. The motion warned of the "disastrous consequences" of repeating the Iraq war.
Aside from the United Nations’ Security Council backing, other conditions include a comprehensive humanitarian plan for displaced refugees, assurances over only Isis being targeted and that diplomacy remains the principle means to end the Syrian civil war".
"Only military action which meets all these objectives, and thus avoids the risk of repeating the disastrous consequences of the 2003 war in Iraq and the 2011 air campaign intervention in Libya, can secure the assent of the British people," the motion states.
Labour MPs are divided over the issue Hilary Benn, the shadow foreign secretary, told the conference on Monday that Labour would support effective action in Syria, but ruled out backing the deployment of UK "boots on the ground".
But Diane Abbott, the shadow international development secretary, told conference she opposed airstrikes.
MPs are likely to vote without being marshalled by party enforcers.
John McDonnell, the shadow chancellor, said last night Labour MPs may be a given a free vote if Mr Cameron seeks a Commons mandate supporting airstrikes in Syria, as seems increasingly likely.
The motion was submitted by the Unite union. A Unite delegate told conference: "This party must force Cameron to pause for thought."