Britain and the United States have declared they have found evidence that chemical weapons have been used in Syria's bloody civil war.
Downing Street described the evidence as "limited but persuasive", but did not say whether it believed the regime or the rebels had been responsible for their use.
The White House said the US intelligence community had assessed with "varying degrees of confidence" that chemical weapons had been used by the regime on a "small scale".
The announcements stepped up the pressure on the regime of Bashar al-Assad, which was previously warned by the US that any chemical warfare would see it pass a "red line".
Administration officials, though, emphasised that the latest findings were not an "automatic trigger" for military intervention, as President Obama had previously threatened.
US Senator John McCain said it is "pretty obvious" that the red line "has been crossed" with the Assad regime's apparent use of chemical weapons.
The Republican urged Mr Obama to provide weapons to the Syrian resistance movements as a result of the information from the US intelligence agencies.
Downing Street and the White House have instead called on President Assad to allow United Nations inspectors to conduct a full investigation to establish what had happened.
A Government source confirmed to ITV News that samples of a material taken from inside Syria had been tested and found to contain the chemical warfare agent Sarin.
The source said the test was conducted at the Government's Defence Science and Technology Laboratory at Porton Down and added: "We need to urgently establish more information."