Foreign Secretary William Hague has said for the first time that the "atrocity" in Syria "was a chemical attack" carried out by Assad forces.
Mr Hague said that the chances of the attack being a conspiracy by opposition forces was "vanishingly small" and that the only "plausible explanation" for the mass deaths was an attack by the regime.
He said it was essential to get UN inspectors already in the country to the site outside Damascus and that "time was of the essence".
If inspectors were not allowed in, the evidence, he said, would deteriorate.
Girls who survived the apparent attack take refuge in a mosqueSyria's opposition said it was "critical that the UN inspectors [get to the site] within 48 hours".
Forces loyal to President Bashar al Assad have been carrying out air strikes on the area, which could already have considerably damaged any proof of the use of nerve gas.
And Mr Hague warned the Assad regime that if they did not comply with the UN request to carry out inspections at the scene of the attack then Britain would be prepared to go back to the UN Security Council for a mandate "for the world to speak more forcefully about this".
A child receives treatment in a make-shift hospitalHe stressed that inspectors were just 20 minutes away from the site where 1,300, including scores of children, are said to have perished on Tuesday.
Mr Hague said: "The only possible explanation of what we have been able to see is that it was a chemical attack and clearly many, many hundreds of people have been killed, some of the estimates are well over 1,000.
"There is no other plausible explanation for casualties so intense in such a small area on this scale.
"I know some people in the world would like to say this is some kind of conspiracy brought about by the opposition in Syria - I think the chances of that are vanishingly small.
"So we do believe this is a chemical attack by the Assad regime on a large scale but we would like the United Nations to be able to assess that so those who don't believe that, those who doubt that, the evidence can be gathered.
Activists inspect the bodies of the victims of the alleged gas attack"But that is certainly our opinion."
He said it "seems the Assad regime has something to hide" and questioned why it had refused to allow the UN inspectors, who are staying in a hotel in Damascus, to investigate.
Footage and images released by the opposition has showed scores of children lying dead, some seemingly gassed as they slept in their beds.
Other pictures appear to show adults foaming at the mouths from the effects of gas and row upon row of corpses, some being loaded into vans.
A map showing the area targeted in the apparent chemical attackThe images, Mr Hague said, were "something that a humane or civilised world cannot ignore".
UN chemical weapons inspectors have been in the country since Sunday on a mission to investigate sites where previous gas attacks are alleged to have been carried out.
Activists said on Friday they were smuggling body tissue samples from victims to the UN inspectors in their hotel but that it had been a struggle because they were heavily guarded by government forces.
One activist, Abu Nidal, said: "The UN team spoke with us and since then we prepared samples of hair, skin and blood and smuggled them back to Damascus with trusted couriers."
In his first comments since the footage of the attacks emerged, US President Barack Obama said it was a "big event of grave concern" and that " this is something that is going to require America's attention".
The chemical weapons inspectors arrive in DamascusUN Secretary General Ban Ki-moon said that the use of chemical weapons would constitute "a crime against humanity".
Significantly, Syria's key ally Russia has joined international calls for the inspectors to be given access to the site of the alleged massacre.
Moscow suggested the attack could be a "premeditated provocation" by opposition forces but urged Mr Assad and the UN to agree to a visit to the site in the Damascus suburbs of Ein Tarma and Zamalka.
But Russia dismissed calls for use of force against its ally.
"Against the background of another anti-Syrian wave of propaganda, we believe calls from some European countries to apply pressure on the UN Security Council and already now take a decision on the use of force are unacceptable," its foreign ministry said.