Britain, France and the US on Thursday tried to pile pressure on an increasingly emboldened Vladimir Putin by producing new evidence that lethal sarin nerve gas was used in the notorious chemical attack in Syria in August.

The predominantly British claims were based on tests of clothing and soil samples that David Cameron said had been taken from Syria and tested positive for sarin by scientists at Porton Down, Wiltshire.

The British prime minister said that "we were confident and remain confident that Assad was responsible" for the attack on Ghouta, east Damascus, and added: "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."

Cameron's fresh evidence was aggressively dismissed by Russia in an acrimonious end to the first day of the G20 summit in St Petersburg, at which events were dominated by the crisis in the Middle East.

A senior spokesman for Putin was reported to have told Russian journalists in a briefing that Britain was "a small island no one listens to", apart from some oligarchs who had bought Chelsea football club. The Russian foreign minister, Sergei Lavrov, insisted that any evidence about the use of chemical weapons in Syria should be presented by the UN security council.

Cameron was forced to dismiss the belittling criticism – initially believed to have been made by the Russian president's press attache, Dimitri Peskov – saying Britain was the leading the argument for a strong response to the use of chemical weapons.

A No 10 source said: "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."

Later, Peskov angrily denied he had made the remarks, broadcast hours before Putin was due to meet Cameron for a late-night discussion. "I simply can't explain the source of that claim … It is definitely not something I have said," he said.

Cameron's frustration at the way in which he has been forced to stand aside from any US-led military action against the Syrian government was revealed when he questioned how Labour could live with itself after 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.

He 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 carried out in the past seven days by British scientists at the Porton Down facility, and were deployed by Cameron in a fresh attempt to persuade the Russian president to do more to force the Syrian president, Bashar al-Assad, to the negotiating table.

The samples brought to Britain from the Syrian borders are different from 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 Britain. The results of the separate UN weapons inspectors' on-site tests in Syria have yet to be revealed, but are likely to be known around the time the US Congress votes on whether to backmilitary 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 G20 agenda, Putin relented on Thursday and said the crisis would be discussed over dinner at the summit. Discussions during the afternoon concentrated on economic matters.

On Thursday night the French led a charge calling for a clear G20 statement condemning the use of chemical weapons. President François Hollande said: "It's really important that the Europeans here at the G20 are on the same page in condemning the use of chemical arms and condemning the regime that used them."

The Italians looked pivotal in determining the European view, with the Germans convinced that the Assad government was behind the chemical attacks. UK sources claimed those condemning Assad far outweighed the Syrian leaders supporters at the dinner.

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."

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.

The prime minister added: "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."

In the opening exchanges of the summit, Putin gathered an impressive array of allies at the G20 urging Barack Obama to delay taking military action at least until there was a clearer picture of responsibility for the attacks.

Enrico Letta, the prime minister of Italy, said the summit was "the last chance to find political solutions" to the Syrian crisis. Others who urged the US to delay military action included the Chinese, the Pope and leaders in the emerging world.

Pope Francis also urged the Group of 20 leaders to abandon the "futile pursuit" of a military solution in Syria as the Vatican laid out its case for a negotiated settlement that guarantees rights for all minorities, including Christians. In a letter to Putin, Francis lamented that "one-sided interests" had prevailed in Syria, preventing a diplomatic end to the conflict and allowing the continued "senseless massacre" of innocents.

The US ambassador to the UN, Samantha Power, accused Putin on Thursday night of holding the security council hostage. Power said: "Russia continues to hold the council hostage and shirks its international responsibilities." She blamed the structure of the security council, which lets five major nations hold veto power Russia, the United States, China, France and Britain.

Obama began personally calling wavering US legislators during his trip to Russia as his pursuit of congressional authorisation for military action against Syria threatened to drag on well into next week.

The majority of Congress remains undecided, according to various unofficial "whip counts" collated in Washington, and the president has cancelled a trip to California planned for next week "to work on the Syrian resolution before Congress", according to a White House official.

Obama is also expected to make a direct to appeal to the American people in a televised address from the Oval office when he returns from the G20 summit in Russia.

Although the White House received a boost on Wednesday when an influential Senate committee narrowly voted in favour of military action, the introduction of clauses calling for regime change by hawkish Republicans has complicated efforts to secure support among more liberal Democrats.

A running tally maintained by CNN estimated 24 senators were preparing to vote yes, while 17 were leaning towards a no vote, with the remaining 59 undecided. In the House, the position appears reversed, with 97 leaning against and only 28 so far declared in favour.

The administration was also put on the back foot on Thursday by a New York Times report detailing alleged atrocities by Syrian rebel fighters. State department spokeswoman Jen Psaki condemned the apparent attack and said the US was seeking more information from rebel commanders.

Meanwhile, Cameron repeatedly insisted he had not 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."

US secretary of state John Kerry will fly to Paris and London this weekend to help shore up international support for action against Syria amid various reports about the size of the coalition aligned with the US.

The state department said at least 80 countries or organisations had acknowledged and condemned the use of chemical weapons in Syria – more than 50 of these publicly. A further 30 or more have said Assad was responsible, according to the state department, and nine were in support of US military action: Australia, Albania, Kosovo, Canada, Denmark, France, Poland, Romania and Turkey. An unspecified number had offered military support of their own, but the US says it has the capacity to act alone if necessary.