Britain would be permitted to take military action against Syria on humanitarian grounds, according to legal advice the Government has published today.
Downing Street has released the legal basis for intervention as the Commons debates a targeted strike even if Russia and China block an agreement at the United Nations.
It has also released evidence from the Joint Intelligence Committee, which found that a chemical weapons attack did occur in Damascus last week and that it is "highly likely" that Bashar Assad's regime was responsible.
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A No10 spokeswoman said: "Today we have laid in the library of the House information on what we know about the attack in Damascus last week and the Government's position on the legality of any military action in response.
"This reflects the PM's commitment to build a consensual approach and to ensure that MPs can properly consider the issues before voting on the UK response.
"The judgment of the Joint Intelligence Committee is that a chemical weapons attack did occur in Damascus last week; that it is highly likely that the Syrian regime was responsible; that there is some intelligence to suggest regime culpability; and that no opposition group has the capability to conduct a chemical weapons attack on this scale.
"The Government's position on the legality of any action makes clear that if action in the UN Security Council (UNSC) is blocked, the UK would still be permitted, under the doctrine of humanitarian intervention, to take exceptional measures including targeted military intervention in order to alleviate the overwhelming humanitarian suffering in Syria.
"The Cabinet endorsed the recommendation from the national security council and the Prime Minister will set out the proposed Government response in the House this afternoon.
"Ministers agreed that it is fundamentally in our national interest to uphold the longstanding convention on chemical weapons and to make clear that they cannot be used with impunity.
"Any response should be legal, proportionate and specifically in response to this attack and everyone around the Cabinet table agreed that it is not about taking sides in the Syrian conflict nor about trying to determine the outcome."
The JIC found there are "no plausible alternative scenarios" to Assad's regime being behind the attack and has given the Prime Minister full access to "highly sensitive" intelligence.
It could not, however, come up with a "precise motivation" for the attack.
In a letter to Mr Cameron, its chairman Jon Day said: "There is little serious dispute that chemical attacks causing mass casualties on a larger scale than hitherto (including, we judge, at least 350 fatalities) took place.
"It is being claimed, including by the regime, that the attacks were either faked or undertaken by the Syrian Armed Opposition. We have tested this assertion using a wide range of intelligence and open sources, and invited HMG and outside experts to help us establish whether such a thing is possible.
"There is no credible intelligence or other evidence to substantiate the claims or the possession of CW (chemical weapons) by the opposition. The JIC has therefore concluded that there are no plausible alternative scenarios to regime responsibility."
The attack on the eastern suburbs of Damascus "amounts to a war crime and a crime against humanity", according to the document setting out the Government's legal position.
But the legal basis for military action would be on humanitarian grounds to relieve the suffering of the Syrian population by "deterring or disrupting the further use of chemical weapons", it adds.
That holds in international law even if the UN Security Council fails to secure an agreement on international intervention as long as there is "convincing evidence" of extreme humanitarian distress, there is no "practicable alternative" to the use of force and and action is "necessary and proportionate".
All three conditions "would clearly be met in this case", a summary of the Government's legal advice states.