Asked about reports that President Bashar al-Assad’s forces had used chemical weapons, which are banned under international law, the Prime Minister replied: “I choose my words carefully, but what I see does look very much like a war crime is being committed in our world, at this time, by the Syrian government.”

Speaking at the White House tonight, US President Barack Obama said the use of chemical weapons in Syria "is going to be a game-changer".

“Horrific as it is when mortars are being fired on civilians and people are being indiscriminately killed, to use potential weapons of mass destruction on civilian populations crosses another line with respect to international norms and international law,” Mr Obama told reporters.

“That is going to be a game changer. We have to act prudently. We have to make these assessments deliberately. But I think all of us ... recognise how we cannot stand by and permit the systematic use of weapons like chemical weapons on civilian populations,” he said.

The latest incidents took place at 1am and 7am on Thursday in the southern town of Daraya, according to the Syrian Support Group (SSG), a Washington-based organisation representing the rebel Free Syrian Army. The regime’s forces fired two “chemical-filled” rockets which released a poisonous gas affecting 105 people.

But the chemical agent involved was probably not the sarin nerve gas allegedly used in earlier incidents, added the SSG. Instead, the symptoms of the victims were similar to those inflicted during an attack on March 19 using an organophosphate called echothiophate.

This substance is not banned under the Chemical Weapons Convention. Echothiophate is a “simulant” of sarin, which has a similarly harmful affect on humans. But it does not meet the legal definition of a chemical weapon.

This raises the possibility that Syria’s regime is testing the boundaries set by a public declaration from Mr Obama that any use of chemical weapons would be a “game-changer”. By using a nerve agent that is not officially defined as a weapon, the regime might be aiming to terrorise its enemies, but without clearly breaching Mr Obama’s red line.

A statement from the SSG said the victims of Thursday’s incidents were suffering from “bronchial spasms, headaches, dizziness, vomiting and miosis”.

Mr Cameron acknowledged that memories of the case for war in Iraq, which was built on false reports of the country’s possession of weapons of mass destruction, would make many people sceptical of any British claim that Syria was carrying out chemical attacks. But he told the BBC: “I would want to reassure people and say the lessons of Iraq have been learned. There are proper processes in place to try and make sure that what people say is properly backed up.”

One soil sample that was smuggled out of Syria tested positive for sarin gas in experiments conducted at Porton Down, the Army’s scientific research centre.

The White House said that Mr Obama was awaiting a “definitive judgment” on whether the regime had used chemical weapons before deciding what action to take. “We’re working to establish credible and corroborated facts,” said Jay Carney, the White House spokesman. “The president wants the facts.” But there was no timetable for reaching a definitive conclusion, added Mr Carney.

Israel’s government said that chemical weapons had been used and Mr Assad had breached America’s red line. “From the moment the international community understands that red lines have been crossed and that chemical weapons have been used, they will realise there’s no other choice than to react,” Zeev Elkin, the deputy foreign minister, told Army Radio. “There is a question here: when a red line is set can we stick by it?”

However, Omran al-Zoubi, the Syrian information minister, denied that the regime had used chemical weapons - or even that it possessed such an arsenal. He said that any such attacks had been carried out by the rebels, telling Russia’s Interfax news agency: “This has been done by organisations, including al-Qaeda, which threatened to use chemical weapons against Syria. They have carried out their threat near Aleppo.”

Tonight, the Syrian state news agency tweeted that the information minister had said "Western sides" were "directly responsible" for a chemical attack in Khan al-Assal, on the outskirts of Aleppo.

Outside experts believe that Syria has the largest arsenal of chemical weapons in the Middle East.

After the first reports of the use of gas earlier this month, America and Britain urged a United Nations inquiry to establish the truth. Mr Assad’s regime then made clear that no international investigators would be allowed to enter Syria.