Britain "cannot allow diplomatic paralysis to be a shield" for President Bashar Assad, Foreign Secretary William Hague has said.
Ahead of a meeting of Britain's Nations Security Council on Wednesday, Mr Hague put forward the Government's case for a military strike against the Syrian regime, saying global security was at stake, threatened by what he called the "creeping normalisation" of chemical weapons.
In a comment piece for the Daily Telegraph, he said: "We cannot allow the use of chemical weapons in the 21st century to go unchallenged. That would send a signal to the Syrian regime that they will never face any consequences for their actions, no matter how barbarous. It would make further chemical attacks in Syria much more likely, and also increase the risk that these weapons could fall into the wrong hands in the future.
"But this is not just about one country or one conflict. We cannot afford the weakening of the global prohibition against the use of chemical weapons. We must proceed in a careful and thoughtful way, but we cannot permit our own security to be undermined by the creeping normalisation of the use of weapons that the world has spent decades trying to control and eradicate."
He said: "This actual, repeated use of chemical weapons in Syria is a moral outrage, a serious violation of international humanitarian law and a challenge to our common security. We are now weighing with the United States and our other allies how to respond in a way that is legal and proportionate. The goal of any response should be to prevent further similar humanitarian distress, to deter the further use of chemical weapons in Syria and to uphold the global ban against their use."
He issued a plea to the United Nations Security Council to "rise to its responsibilities by condemning these events and calling for a robust international response", adding: "But all previous attempts to get the Security Council to act on Syria have been blocked, and we cannot allow diplomatic paralysis to be a shield for the perpetrators of these crimes."
Mr Hague warned that "decades of painstaking work to construct an international regime of rules and checks, overseen by the UN, to prevent the use of chemical weapons and to destroy stockpiles" would be undone if the international community failed to act.
"Over the past year we have seen evidence of the repeated small-scale use of chemical weapons by the Syrian regime," said Mr Hague. "We know this from physiological samples that have been smuggled out of Syria and from other sources of information."
He added: "We strongly support the work of the UN team on the ground in Syria. We hope that the information they obtain will help build a fuller picture of the attack - adding to the evidence which already exists - and to help ensure that those responsible for this war crime are held accountable."