The international community cannot "contract out" its morality by allowing Russia to block intervention in crises such as that engulfing Syria, David Cameron has said.
As the G20 summit in St Petersburg wrapped up without consensus on the key issue, the Prime Minister insisted military action had to be possible without an endorsement from the UN Security Council.
He also stressed that backers of direct reprisals against the Syrian regime had made a "powerful" case at the G20 summit, despite failing to overcome president Vladimir Putin's opposition.
Mr Cameron said a strong response against Bashar Assad was necessary to preserve the "international taboo" against chemical weapons.
"This summit was never going to reach agreement on what action is needed in Syria," he said. "But the case made by those countries who believe we have to take a stand against the use of chemical weapons was, I believe, extremely powerful."
He said he believed the countries calling for action had the "better of the argument" during the two-day summit. "If we are saying that there can only be a response if the UN security council votes positively we are in fact contracting out our foreign policy, our morality to the potential of a Russian veto," he added.
Mr Cameron again defended his decision to hold an early Commons vote on the principle of military intervention - even though he suffered a shock defeat. "I believe I was right to take a strong stance on chemical weapons and to take that to the British parliament," he said. "But I also understand and respect what the UK parliament has said, so Britain will not be part of any military action."
Summit host Mr Putin has rallied opposition from a number of countries at the summit to Barack Obama's proposals for punitive action against Assad, with only France suggesting it will join in any military strikes.
Mr Cameron has confirmed that deep divisions over Syria were voiced at last night's four-hour official dinner and said that - despite evidence of nerve gas sarin found by US and UK scientists - Mr Putin is still far from accepting the regime's responsibility for the August 21 attack which killed hundreds of civilians in a Damascus suburb.
The premiers of Turkey, Canada, Germany and Italy all joined Mr Obama and Mr Cameron in making the case for a robust international response to Assad's alleged breach of treaties banning the use of chemical weapons. But it is understood that Mr Putin's argument that any action must be approved by the UN Security Council was backed by several countries, including China. Permanent Security Council member Russia has repeatedly blocked proposed sanctions against the Assad regime during the course of the two-and-a-half year civil war in Syria.