David Cameron has been given a legal green light to launch military action against Syria - even without the United Nations' approval.
Official legal advice for the government said intervention would be permitted "in order to alleviate the scale of the overwhelming humanitarian catastrophe" caused by chemical weapons.
It comes as intelligence officials concluded it was "highly likely" that President Assad's regime had been responsible for the Damascus chemical attacks that shocked the world.
The legal position was revealed when Downing Street published its legal advice for intervention ahead of an emergency Commons debate about the crisis on Thursday afternoon.
It says: "If action in the Security Council is blocked, the UK would still be permitted under international law to take exceptional measures in order to alleviate the scale of the overwhelming humanitarian catastrophe in Syria by deterring and disrupting the further use of chemical weapons by the Syrian regime."
Precedents for military intervention without UN approval include Kosovo, which was deemed to be a success, and of course Iraq, which dogged Tony Blair and Labour for the following decade.
Meanwhile, the Joint Intelligence Committee said it had "concluded that there are no plausible alternative scenarios to regime responsibility."
This was not enough for some critics of intervention.
Tory MP Julian Lewis told the BBC: "The JIC summary is inconclusive".
A Number 10 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."
Labour, however, will vote against the Government's motion on the principle of military intervention in Syria, a senior party source said told the Press Association on Thursday afternoon.