Cameron said scientists at Britain's Porton Down military research facility had analysed samples taken from an alleged gas attack on a rebel-held Damascus neighbourhood on August 21 and concluded they had tested positive for the sarin nerve agent.
"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," he told BBC TV.
Last week Cameron lost a parliamentary vote backing military action to punish Syrian President Bashar al-Assad for the reported chemical attack, which Washington says killed more than 1,000 people.
But the British leader denied this had left him on the sidelines of talks at a meeting of the leaders of the Group of 20 (G20) developed and developing economies in Russia.
"Not in the slightest," he said when asked if he had no hand to play at the G20 summit, adding Britain would be "leading the argument on humanitarian aid".
Cameron also said U.S. President Barack Obama was right to press ahead with possible military strikes against Syria as Assad had ignored his warnings about using chemical weapons during the country's 2-1/2-year-old civil war.
"I absolutely believe that, having set a red line on the further big use of chemical weapons, it would be wrong if America was to step back and, having set that red line, to do nothing," he said. "I think that would send an appalling signal to President Assad and also to dictators elsewhere."
