Any resolution at the 15-strong Security Council can be vetoed by any of the five permanent members - the UK, the US, France, Russia and China.
Earlier today, the government said it would be legally justified to act unilaterally on a humanitarian basis without the backing of the UN. Cameron's comments today suggest the UK and the US could go ahead with intervention, even if Russia and/or China were opposed.
Cameron would still wait for the results of the UN inspection and would need agreement in the Commons before proceeding.
Speaking in the Commons today:
It cannot be the case that that is the only way to have a legal basis for action. We should consider what the consequences would be if that were the case. You could have a situation where a state was annihilating literally half of its population... with one veto on the security council you would be hampered from taking action... We do have the doctrine of humanitarian intervention that is set out in the Attorney General's excellent advice to this House...
The question before us is: 'Is Britain a country that wants to uphold that international taboo against chemical weapons?'.