British scientists 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.
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.
The Prime Minister was forced to deny 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.
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.
Dismissing as "complete nonsense" suggestions that the episode had damaged the UK-US special relationship, Mr Cameron said: "The relationship between Britain and America is so good that we don't have to measure it in the number of telephone calls or meetings."
Deputy Political Editor Chris Ship reports from St Petersburg:
A Downing Street source also defended Britain amid reports that an aide to the Russian president dismissed Britain as "just a small island; no one pays any attention to them".
The source told the Press Association: "As host of guests from the world's leading countries, I'm sure the Russians will want to clarify these reported remarks, particularly at a G20 where it's a very British agenda on trade and tax.
"It highlights how a small island with great people can achieve a big footprint in the world."
Mr Putin's spokesman Dmitry Peskov denied making the comment, which was attributed to him by the BBC.