British scientists at Porton Down have found positive evidence of deadly sarin gas on samples of cloth and soil from the Damascus suburb believed to have been targeted by forces loyal to Syrian president Bashar Assad.
The UK's confirmation of poison gas use will further fuel calls for action against Assad at the G20 summit in St Petersburg, where host Vladimir Putin has agreed to put Syria on the agenda for talks over dinner tonight.
David Cameron said the test result "adds to the picture" of chemical weapons use, but said the key issue was whether those who question the regime's culpability - including the Russian president - can be convinced Assad is to blame.
Mr Cameron denied he had been sidelined at the summit by his decision to rule out British involvement in military action following his shock defeat over Syria in the House of Commons last week.
He played down the significance of US president Barack Obama holding bilateral talks with the leaders of France and Japan in St Petersburg, but not with America's traditionally strongest ally Britain.
In a round of TV interviews in St Petersburg, Mr Cameron said: "I absolutely believe that having set a red line on the further big use of chemical weapons, I think it would be wrong if America was to step back and, having set that red line, to do nothing. I think that would send an appalling signal to President Assad and also to dictators elsewhere.
"The House of Commons decision, as I interpret it, is that there should be no British involvement in that military action, and I respect that. But the world does need to respond strongly and I won't stop making that argument."
Britain is to brief the United Nations on the discoveries made by the Defence Science Technology Laboratory at Porton Down. It is understood that the materials under examination came via a third country in the region and were reported to have come from the Ghouta neighbourhood where hundreds of civilians died on August 21.
Mr Cameron said: "I think the evidence is growing all the time. We have just been looking at some samples taken from Damascus in the Porton Down laboratory in Britain which further shows the use of chemical weapons in that Damascus suburb...
"I think we will add to that evidence. For some people there will never be enough evidence and for some in the debate in the House of Commons it wasn't about evidence, it wasn't about chemical weapons, it was about how they felt let down over Iraq and a deep concern - which I completely understand - about not wanting to get further involved in the difficulties in Syria. My view is we have to look at chemical weapons as something different, something awful. Those pictures of children being gassed on our television screens are something the world must not turn away from."