"We do believe that this is a chemical attack by the Assad regime on a large scale," Foreign Secretary William Hague said today.
"It is now 48 hours since the reports started to come in of what seems to have been a terrible atrocity near Damascus including the use of chemical weapons.
"This is not something that a humane or civilised world can ignore," he said. "The only possible explanation of what we've been able to see is that it was a chemical attack."
He said the UN's priority was to allow inspectors onto the sites of the attacks but so far that had not been allowed.
"Already it seems that the Assad regime has something to hide - why else wouldn't they allow the UN team to go there?"
Hague said he hoped to speak to the Russian foreign minister later today.
Britain has directly accused the Assad regime of gassing hundreds of Syrian civilians amid warnings only 48 hours remain to find proof.
New videos show the eyewitness accounts of four men who witnessed an alleged chemical attack on Zamalka, a suburb of Damascus.
These are the images of the remains of rockets which, according to the men stood by them, delivered poisonous gas to a suburb of Damascus.