The fact-finding mission by chemical weapons watchdog Organisation for the Prohibition of Chemical Weapons (OPCW) concluded Sarin, a banned nerve agent, was used in the attack on Khan Sheikhoun in Idlib province.
The attack on Khan Sheikhoun in Idlib province was the deadliest in Syria in more than three years. It prompted a retaliatory US missile strike against a Syrian air base the US said had launched the attack.
The OPCW, which is based in The Hague, reached that conclusion after interviewing witnesses and examining samples.
"A large number of people, some of whom died, were exposed to Sarin or a Sarin-like substance," the report said.
"It is the conclusion of the FFM (fact-finding mission) that such a release can only be determined as the use of Sarin, as a chemical weapon," a summary said.
The new report has been circulated among OPCW members but has not been made public.
A joint UN and OPCW investigation will now investigate who was to blame for the attack.
US President Donald Trump ordered a cruise missile strike on Shayrat air base after US officials concluded that the facility was where a Syrian Air Force jet had been armed with a Sarin-filled bomb.
Syrian President Bashar al-Assad has previously said that the incident was fabricated.
Photo: AFP A picture taken on 4 April, 2017 shows destruction at a hospital in Khan Sheikhun, a rebel-held town in the northwestern Syrian Idlib province, following a suspected toxic gas attack.