A chemical weapons watchdog investigation has confirmed the nerve gas sarin was used on the attack on Khan Sheikhoun in Syria on April 4.
The report by the Organisation for the Prohibition of Chemical Weapons (OPCW) does not identify who was responsible, but Foreign Secretary Boris Johnson said there was "absolutely no doubt that the finger points at the Assad regime."
He said: "There is abundant evidence that that it is exactly the same sarin that we know to have been in possession of the Assad regime."
The report will now be passed on to the UN / OPCW Joint Investigative Mechanism to work on identifying who exactly was responsible.
At least 86 people, including 30 children, died and dozens more were injured in the attack.
Theresa May and Donald Trump were among world leaders blaming President Assad for the "barbaric" attack. Russia continued to back Assad and claimed the attack was caused by a Syrian air strike which hit a stockpile of chemical arms.
The Syrian regime denies dropping chemical weapons.
Speaking about the report, Mr Johnson said: "This is the first step in the process that the UK has been trying to lead to try to hold to account the people who were responsible for dropping chemical weapons on Khan Sheikhoun on April 4th.
"I'm pleased, though not surprised, that the OPCW (Organisation for the Prohibition of Chemical Weapons), confirmed that this is indeed sarin."
He added: "People who drop chemical weapons should be held to account."
Asked if he thought the report would have an effect, Mr Johnson said: "I think it is having an deterrent effect. What's most interesting and important of all, is that you're starting to see a certain Russian impatience with their satellite, with their client Assad, a recognition that he really embarrassed Moscow by allowing his generals to do this.
"And that is one of the objectives we're trying to achieve, to drive a wedge between the Russians and Iranians, and the Russians and Assad."