A report by UN weapons inspectors into the atrocity in Damascus is "damning" and "fully consistent" with Britain's assessment that Bashar Assad's regime was behind the attack, William Hague said.
The foreign secretary spoke out after UN secretary general Ban Ki-moon presented what he said was "overwhelming and indisputable" proof that chemical weapons were used on a large scale.
Although the team was not mandated to establish who was responsible for the use of the banned weapons, Mr Hague said their findings backed the West's claims it was government forces.
"This report, which we are analysing in detail, is clearly very damning," Mr Hague said.
"It confirms that there was indeed a large-scale chemical weapons attack on the areas east of Damascus in the early hours of 21 August.
"It confirms that this was an attack against civilians, against children and a large number of people were killed and it is fully consistent with everything we have always argued about this attack - that sarin was used, that it was on a large scale."
He added: "We have always believed that this was the work, the responsibility of the Assad regime and everything we can see in this report is fully consistent with that."
Prime Minister David Cameron said the report was "chilling".
"No-one can ignore facts," he wrote on Twitter, adding: "100s gassed in worst attack since Halabja. UNSG right: we must destroy chemical weapons."
UN secretary general Ban Ki-Moon called the attack a 'despicable war crime'
Mr Hague said he was "hopeful" of an international deal for Syria to give up its chemical weapons but warned it would be a hugely challenging process.
He and French counterpart Laurent Fabius were briefed in Paris by US secretary of state John Kerry earlier on his deal with Russia for the international community to take control of the regime's stockpile.
"Yes we are hopeful, but very mindful of all the difficulties of identifying and securing probably the largest arsenal of chemical weapons in the world, in a country that is a contested battlefield," Mr Hague said.
"No-one should underestimate the challenge of that."
Presenting the report, Mr Ban said: "The results are overwhelming and indisputable. The facts speak for themselves," he said.
They had "collected clear and convincing evidence that surface-to-surface rockets containing the nerve agent sarin were used in the Ein Tarma, Moadamiyah and Zalmalka in the Ghouta area of Damascus".
Mr Ban said: "The United Nations mission has now confirmed, unequivocally and objectively, that chemical weapons have been used in Syria.
"This is a war crime and a grave violation of the 1925 Protocol and other rules of customary international law. I trust all can join me in condemning this despicable crime.
"The international community has a responsibility to hold the perpetrators accountable and to ensure that chemical weapons never re-emerge as an instrument of warfare."
Environmental, chemical and medical samples collected in the aftermath of the assault last month on a rebel-held Damascus suburb which is estimated to have killed up to 1,400 people show that rockets deployed in the attack contained nerve gas sarin, the team has reported.
Mr Hague said: "It is the Assad regime that has stockpiled these weapons and that has used them repeatedly against the Syrian people.
"So the pressure is on them to comply with this agreement in full. The world must be prepared to hold them to account if they don't and our three countries are certainly determined to do so."
After the Paris talks, Mr Hague said the UK's first priority was to secure prompt action at the UN Security Council that enshrines in a resolution the Syrian regime's responsibility to hand over its chemical weapons stocks.
He added the resolution should create a binding commitment for the regime to give up its weapons within a specific time-frame and to "credibly, reliably and promptly" place them under international control for destruction.
Under the agreement hammered out between Mr Kerry and Russian foreign minister Sergei Lavrov in Geneva, the regime is required to submit a full inventory of its chemical stockpile by the end of the week, allow international inspectors into the country by November, and complete the surrender of its arsenal by mid-2014.
Mr Hague said the allies are in agreement that the "credible threat of military force" played a role in bringing about an opening in relation to Syria and insisted the UK, US and France would use their "full weight" as permanent members of the UN Security Council to get the chemical weapons out of Syria and bring about a peaceful end to the brutal civil war.
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