Cameron: British scientists have proof deadly sarin gas was used in chemical weapons attack

Samples from soil and victim's clothing provide independent confirmation that illegal weapon was used against civilians and opposition fighters

David Cameron confirmed that Sarin had been used in chemical attacks (EPA)

British scientists have detected deadly sarin nerve agent on the clothing of one of the victims of the gas attack in Syria, David Cameron revealed today.

Samples collected from the clothes of a victim of the 21 August attack in Damascus that killed hundreds of men, women and children were tested positive at the secret Porton Down laboratories.

Soil at the site of the massacre also contained sarin traces. It means independent British confirmation that one of the cruellest of illegal weapons was unleashed against civilians and opposition fighters.

But Barack Obama’s call for attacks to punish Syrian dictator Bashar al-Assad was jolted when the Pope warned against “futile” military strikes.

In a boost for Russian leader Vladimir Putin who is leading opposition to military action at the G20 summit, the leader of the world’s 1.2 billion Roman Catholics said attacks would not solve the crisis.

“Rather, let there be a renewed commitment to seek, with courage and determination, a peaceful solution through dialogue and negotiation of the parties, unanimously supported by the international community,” Pope Francis said in a letter to Mr Putin, the summit host.

In his first big intervention on the global stage since he became Pope in March, Francis appealed: “To the leaders present, to each and every one, I make a heartfelt appeal for them to help find ways to overcome the conflicting positions.

He called on Catholics to join him on Saturday in a day of prayer and fasting to end the Syrian conflict.

British confirmation that sarin was used, probably by Assad forces, will reinforce the case for missile strikes.

Sarin is a clear liquid that turns into an odourless vapour – and is far more deadly than cyanide.

Arriving at St Petersburg for a G20 summit dominated by the question of whether to punish Bashar al-Assad’s regime for an apparent act of mass murder, the Prime Minister issued an urgent call for drugs and equipment to help Syrian people survive any further sarin attacks.

He urged to set up a supply of antidote medicines, decontamination tents and other equipment to protect opposition-held areas feared to be in danger.

Today’s announcement comes too late to influence last week’s Commons debate on military action, where Mr Cameron was defeated by just 13 votes and forced to rule out taking part.

It will fuel criticism of the Government for rushing into a vote before the full evidence was available.

A tiny drop of sarin the size of a pinhead can kill in under two minutes, by paralysing lung functions. Victims suffer suffocation, vomiting and diarrhoea; and convulsions, typically dying in 15 minutes unless drugs are administered.

America and France have already announced that their own experts found evidence of banned sarin. Downing Street said the samples tested at the Defence Science Technology Laboratory [DSTL] were obtained independently of the two other countries.

No 10 would not reveal if the victim was male, female, adult or child, or civilian or fighter, saying only that he or she was “a victim”. Neither would sources reveal how British investigators came to possess the samples of clothing.

Speaking at the summit, Mr Cameron said he was “confident that Assad was responsible” for the 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.”

Despite ruling out British participation he urged President Obama not to shrink from attacking Syria for fear of sending the wrong signal to dictators tempted to use banned weapons.

“Having set a red line on the further big use of chemical weapons it would be wrong if America were to step back ... do nothing. That would send a signal to Assad and also to dictators everywhere,” he told the BBC.

He denied the special relationship between Britain and the US had been damaged, although eyebrows were raised that President Obama is having one-to-one talks with French president Francois Hollande but not with Mr Cameron.

“The relationship between Britain and America is so good we do not have to measure it by the number of telephone calls or meetings,” he told ITV.

America says sarin nerve gas killed more than 1,400 people in the attack last month that triggered Barack Obama’s call for military action to punish and degrade the Assad regime. British intelligence says at least 350 people died in the attack, which was “highly likely” to have been carried out by Assad.

Mr Cameron also warned of the “worst refugee crisis this century” as he arrived at the St Petersburg talks and called on fellow leaders to give more money.

The Prime Minister, denying he was sidelined, was using the G20 to focus on humanitarian issues and to rattle the tin for more aid, challenging leaders to match the £348 million being pumped in by Britain alone.

He also wants “safe routes” to safeguard aid convoys from being caught up in the civil war.

"We are facing the worst refugee crisis of this century and millions of lives are being destroyed inside in Syria by Assad and his regime,” the Prime Minister said.

“The world needs to do more to help the innocent victims of this conflict who dreamt of a democratic and peaceful future but who are now living a nightmare far from their homes and struggling to feed their families and keep them safe.”

“I will be using the G20 to ensure their needs are heard and to ensure the international community responds. We must make more money available for aid agencies to help ease the suffering and we must put pressure on both sides in the conflict to improve access so aid workers can get to those who most need help.”

Mr Cameron’s key calls are for donor countries to fill a massive funding gap identified by humanitarian agencies. He will signal Britain will lead by example and put more money on the table but that we want others to do more too.

Pressure on Assad to give safe passage to humanitarian workers and aid convoys is seen as vital to easing the refugee crisis. As is cutting on custom rules and red tape which make it hard for aid workers to deliver swift relief on the ground.

Some 6.8 million people inside Syria need humanitarian assistance, the UN estimates – equivalent to the entire population of Scotland, Birmingham and Manchester combined.