David Cameron has said British laboratory tests have confirmed that the lethal sarin nerve gas was used in the notorious chemical attack in Syria in August.

Speaking at the start of G20 summit in Saint Petersburg, the prime minister also questioned how Labour could live with itself for taking "the easy political way out" in last week's Commons vote, a decision that he said had been taken in the knowledge that children had been gassed to death in eastern Damascus.

Cameron said he took "full and personal responsibility for the decision to recall parliament, for the decision to take a strong and principled stand against the gassing of children in Syria, and I take full responsibility for putting forward as generous a motion I could, to bring as many people with me as I could. Everyone who voted has to live with the way that they voted."

The positive tests for sarin were completed this week and made on clothes and soil taken from the site of the attack in Ghouta, eastern Damascus on 21 August. The tests were carried out in the past seven days by British scientists at the Porton Down facility, and will be deployed by Cameron in a fresh attempt to persuade the Russian president, Vladimir Putin, to do more to force the Syrian president, Bashar al-Assad, to the negotiating table.

The samples brought to the UK from the Syrian borders are different to the hair and blood samples tested in the US. Details of those test results were released by the US secretary of state, John Kerry, four days ago.

British sources did not give further details of the precise content of the UK tests, but said they were confident the samples had not been tampered with during their passage to the Syrian border, and then to the UK.

The results of the separate UN weapons inspectors' onsite tests in Syria have yet to be revealed, but are likely to be known around the time the US Congress votes on whether to back military action next week. British intelligence has already produced an assessment that it does not believe the rebel forces had the capacity to mount a chemical attack, but Putin has been arguing that this is the case.

After refusing to put the issue of Syria on to the agenda, Putin relented on Thursday and said the crisis would be discussed over dinner at the summit. Discussions during the afternoon had concentrated on economic matters.

Cameron said: "We were confident and remain confident that Assad was responsible not only for this chemical weapon attack, when we saw children being gassed on our television screens, but also we know that there have been at least 14 previous chemical weapons attacks.

"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."

But he expressed scepticism that the fresh evidence might be "a game changer" in what is becoming an increasingly bitter international debate on whether to regard the chemical attack as a red line that the international community cannot allow to go unpunished.

Cameron said: "All the testing that's been done, including the testing we are doing at our Porton Down laboratories, all adds to the picture. But I don't think anyone is seriously denying that a chemical weapons attack took place. I think the Russians accept that. Even the Iranians accept that.

"The question is obviously convincing more people that the regime was responsible."

German intelligence this week claimed to have intercept evidence revealing the Syrian military had been behind the attacks, but there is little irrefutable material about the provenance of the attack in the public domain.

Cameron repeatedly denied he had been sidelined by the Commons vote last week to reject UK involvement in military action in Syria. He vented his fury at the way in which Ed Miliband, the Labour leader, had handled the vote last week. He said: "My only regret about what happened last week is that, having produced a motion in parliament that was clear about going to the UN, that was clear about listening to the weapons inspectors, that was clear about having another vote before military action – all things that the opposition asked for – that even in spite of that, in my view, they chose the easy and political path, not the right path."

He said he recognised that some MPs had become isolationist: "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."

He expressed his scepticism that Russia was ever going to distance itself from Assad at the United Nations. He said: "I haven't yet seen any evidence that Russia will change their position at the United Nations. But the best way of doing this, of course, is having a UN resolution, a condemnation of Syria, backing for all necessary measures to be taken. That's the best way to do this."

He insisted the British decision to cut off the option of supporting military action had not led to a freezing in the special relationship with America. He said: "When I spoke to President Obama on Friday night he was very understanding. Like me, he takes a strong line on the use of chemical weapons and these appalling things that are happening. Like me he is a democrat who believes you have to listen to people, you have to listen to parliament and you have to respect the outcome of a vote."

Some ministers have conceded that they were pushed into a hasty Commons vote by the Americans without preparing the ground. They point to the contrast of the US Congress where Obama aides have been "flooding the zone" to persuade representatives to back him next week. Cameron said he respected the vote, saying: "Britain isn't going to be involved in this specific military action, but the special relationship between Britain and America is as strong today as it was a week ago."