David Cameron insists Britain has "strong legal advice" that suggests military intervention in Syria would be lawful even without a UN resolution.
"The argument that did flare up at the dinner last night is a disagreement again about whether it is possible to have legal military engagements outside having a UN Security Council resolution," said the Prime Minister.
"Our strong legal advice is that there is a responsibility to protect; there is a case for humanitarian intervention because you are preventing a humanitarian catastrophe.
"There was an argument from some - which I don't think is particularly strong - that unless it is self-defence or unless there is a Security Council resolution, there is no legal basis for taking action. I don't think that is the case.
"I think it is a very dangerous doctrine. If you accept that, you could have a country massacring half its people, a blockage at the UN Security Council, and no one could act.
The G20 summit in St Petersburg exposed stark divisions between host Vladimir Putin and US President Obama over Syria.
War emptied the suburb of Yarmouk in southern Damascus of its people. Assad's army holds it, but their prize is a ghost town.
Even in the President's heartland of Baltimore, 95% of voters are against his policy on military intervention in Syria.