Inspectors from the Organization for the Prohibition of Chemical Weapons concluded in a report released that a toxic chemical containing chlorine was used in last year's attack in Douma.
The head of the global chemical weapons organisation on Monday defended the agency's conclusion that poison was used in a high profile attack in Syria last year after leaked documents suggested two former employees doubted some of its findings.
More than 40 people were killed in the April 7 attack in Douma, a town on the outskirts of Damascus that was then held by rebels.
The US, Britain and France retaliated a week later by firing missiles at Syrian government targets, the biggest Western military action against the Damascus authorities of the eight-year war.
Inspectors from the Organization for the Prohibition of Chemical Weapons concluded in a report released on March 1 that a toxic chemical containing chlorine was used in an attack. The team was not mandated to assign blame for who used the toxic chemical.
However, the Syrian government and its Russian allies have rejected the findings, saying they believe the incident was staged by rebels and no attack had taken place.
On Saturday, anti-secrecy group Wikileaks published an internal email to the former chief of staff at the OPCW in which an unidentified inspector described a report as having been edited to appear more conclusive than the inspectors' findings.
"I am requesting that the fact-finding report be released in its entirety as I fear this redacted version no longer reflects the work of the team," the inspector wrote. An OPCW source said that the June 22, 2018 email was genuine.
OPCW chief Fernando Arias said the body stands by the findings it published in March this year.
"While some of these diverging views continue to circulate in certain public discussion forums, I would like to reiterate that I stand by the independent, professional conclusions" of the report, he told delegates on Monday.
"The secretariat has, as it always does, considered and taken into account all information submitted," he said.
Douma is expected to be a major theme at the annual conference of the OPCW being held in The Hague this week.
Criticism of the OPCW's findings over the incident have been circulating since reports on pro-Russian and pro-Syrian media outlets cited another leaked internal document by a former OPCW employee named Ian Henderson in May.
Henderson, who helped the OPCW's team collect samples during a field mission to Douma, wrote that two cylinders found at the scene were most likely placed there rather than being dropped from the air.
The Syrian war has split the OPCW, once largely a technical organisation, along political lines, with Russia and its ally Syria on one side and the US, France and Britain on the other.
A United Nations-OPCW Joint Investigative Mechanism previously carried out the task of assigning blame for chemical weapons attacks, but Russia vetoed a resolution to extend its mandate beyond November 2017.
The JIM concluded in a series of reports that the Syrian regime forces used both nerve agent sarin and chlorine as weapons, while Islamic State insurgents had used sulphur mustard gas on the battlefield.
A new agency unit, the Investigation and Identification Team, is looking into who was responsible for the Douma attack, against the wishes of Moscow and Damascus.