The UK has fresh evidence of the use of chemical weapons in Damascus, David Cameron has told the BBC as he arrives in Russia for the G20 summit.
Scientists at the Porton Down laboratories have been “examining samples” from Syria’s capital, he said.
The Ministry of Defence has confirmed reports that UK scientists found traces of sarin gas on cloth and soil samples.
Mr Cameron also denied claims he had “no hand to play” over Syria after his Commons defeat on UK military action.
He said the UK would lead calls for more action on aid for refugees and push for fresh peace talks.
In an interview with BBC Political Editor Nick Robinson, Mr Cameron said he took “full and personal responsibility” for the decision to recall Parliament and could not have guaranteed MPs a say in whether Britain should take part in US-led action had he not acted so swiftly.
His “only regret” was that Labour, which voted with Tory and Lib Dem rebels against the government motion, had chosen “the easy and the political path not the right and the difficult path”.
Asked if he had no hand to play at the St Petersburg summit, which is expected to be dominated by discussions about Syria, he said: “Not in the slightest.”
Mr Cameron added: “Britain will be leading the argument on humanitarian aid. Britain will be one of the leaders in bringing forward plans for a peace process for Syria. Britain will be leading the argument across the globe for continuing to respond strongly on chemical weapons.”
He said: “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. I think that would send an appalling signal to President Assad and also to dictators elsewhere.”
The Damascus government is accused of using chemical weapons against civilians on several occasions during the 30-month conflict – most recently on a large scale in an attack on 21 August on the outskirts of the capital.
The government of Syrian President Bashar al-Assad has denied involvement and said the rebels were responsible.
But Mr Cameron said evidence of the Assad regime’s use of chemical weapons was “growing all the time” and scientists at the UK’s chemical warfare research laboratories were investigating the attacks on 21 August.
“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 Nick Robinson.
A Downing Street spokesman later added: “UK experts at Porton Down have tested samples from a victim, reportedly treated as a result of that attack. Both the clothing and soil sample tested positive for sarin.”
Sarin gas is odourless, tasteless and colourless. It attacks the nervous system, often causing respiratory failure, and can cause death within minutes of exposure.
Syria is not officially on the G20 agenda in St Petersburg, which is meant to be about global economic recovery, but it is expected to dominate informal meetings.
US President Barack Obama has begun informal talks with other leaders as he pushes for military action over Syria’s alleged use of chemical weapons.
But Russian President Vladimir Putin has warned that military action without UN approval would be “an aggression”.
Earlier, former UK Defence Secretary Liam Fox said the government’s Commons defeat on military action had left Mr Cameron “sidelined”.
The Conservative MP said the prime minister had been “sent into negotiations on Syria with no hand to play and that was bad for the UK”.
Speaking on the BBC’s Daily Politics programme, he said the vote had “given pause for thought amongst our allies about the deliverability of any promise that future prime ministers might make”.
He added that Parliament needed to reflect on how the events of last week had been interpreted internationally.
Former Lib Dem leader Sir Menzies Campbell said: “If the G20 does not agree about military action against the Assad regime, it can surely unite around a determined effort to improve humanitarian relief.
“The summit would be a qualified success if it could agree to do everything possible to achieve a ceasefire so that the humanitarian effort can proceed unhindered.
“David Cameron should make this a priority.”
Meanwhile, Foreign Secretary William Hague has insisted the UK remains “fully behind” the Syrian opposition after holding talks with its leader in London.
He and Syrian National Coalition president Ahmad Al-Jarba discussed ways Britain could provide “further non-lethal support” for the struggle against the Assad government.