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.