Britain believes there is a "growing body of persuasive evidence" that the regime of President Bashar Assad is using chemical weapons against rebels in Syria, Prime Minister David Cameron's official spokesman has said.
The spokesman said it was important, though, for the independent United Nations (UN) investigation to establish the "full facts" about allegations of chemical weapons use before deciding what the international response should be. He urged the Assad regime to co-operate fully with the inspectors appointed by UN secretary general Ban Ki-Moon
Speaking to reporters at a regular Westminster briefing, he declined to give details of how material obtained from the site of alleged chemical attacks in Syria was brought out of the country for analysis at the Government's Porton Down laboratories.
Britain's ambassador to the UN Sir Mark Lyall Grant told reporters in New York on Tuesday that Britain had evidence suggesting the Assad regime has used various chemical agents, including deadly nerve gas sarin. Similar claims have been made by France.
"It is relatively small quantities but nonetheless repeated use, and any use of chemical weapons is abhorrent," said Sir Mark, who added that the UK has reported a number of incidents to Mr Ban and the UN investigation team.
News of the evidence of chemical weapon use came as the rebel-held border town of Qusair fell to Assad's troops after three weeks of intense fighting, which has seen Hezbollah fighters from neighbouring Lebanon join the battle on the regime's side.
Asked whether Mr Assad should now allow humanitarian agencies access to Qusair to help civilians injured and left homeless by the fighting, Mr Cameron's spokesman said: "The safety and security and wellbeing of the Syrian population is at the heart of the international community's entire approach. The killing and the wounding must stop."
US president Barack Obama has repeatedly said that Syria's use of chemical weapons, or the transfer of its stockpiles to a terrorist group, would cross a "red line" that could trigger further international action.
Asked whether the new evidence was the "game-changer" which could lead to a change in approach towards Syria, Mr Cameron's spokesman said: "What we have done is shared the information and evidence that we have to the UN and we continue to support and assist the UN investigation, which is what is needed to establish the full facts.
"It sends out a very clear message to the Assad regime that it must and should co-operate with that UN investigation."