Environmental, chemical and medical samples collected in the aftermath of the assault last month on a rebel-held Damascus suburb show rockets deployed in the attack contained nerve gas sarin, the team has reported.
The announcement came as the UK joined France and the United States to warn that the world must hold Syria to account if President Bashar al Assad fails to comply with a proposal to destroy the country's chemical weapons.
Foreign Secretary William Hague and French counterpart Laurent Fabius were briefed in Paris by US Secretary of State John Kerry on his deal with Russia for the international community to take control of the brutal regime's stockpile.
Mr Hague said: "It is the Assad regime that has stockpiled these weapons and used them repeatedly against the Syrian people.
"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."
The report by the UN inspectors, led by Ake Sellstrom, set out evidence of the use of chemical weapons rather than who was responsible for deploying them.
UN secretary general Ban Ki-moon said last week the Assad regime had "committed many crimes against humanity".
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 said 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 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 were in agreement that the "credible threat of military force" played a role in bringing about an opening 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.
He said: "We are determined to do everything we can to stop the bloodshed in Syria, to support the moderate opposition and to alleviate humanitarian suffering.
"Our goal remains to convene a second Geneva conference to bring all sides together to agree a solution to the conflict and we will work with Russia on bringing that about as soon as possible.
"In that regard I welcome the (Syrian) National Coalition's decision to appoint an interim prime minister, Ahmed Tohme. There can be no peaceful settlement in Syria without the legitimate Syrian opposition.
He admitted the task ahead was very difficult, but said the countries were determined to use their weight in the UN Security Council to implement the agreement, keep the pressure on Mr Assad and bring about an end to the war.