The head of the world’s chemical weapons watchdog has defended its conclusion that chlorine was used in an attack in Syria in April 2018, after a whistleblower alleged the report misrepresented some of the facts amid Russian claims that the watchdog is being politicised by the west.

WikiLeaks at the weekend published an email from a member of the fact-finding team that investigated the attack which accused the body of altering the original findings of investigators to make evidence of a chemical attack seem more conclusive.

Assad regime used chlorine as a chemical weapon, says US Read more

After the attack on 7 April 2018 in the suburb of Damascus, the US, France and Britain launched missile attacks on suspected chemical weapons facilities run by President Bashar al Assad’s army. Russia and Assad have always denied a chemical attack occurred, saying it was probably staged by UK intelligence.

Alexander Shulgin, Russia’s ambassador to the Organisation for the Prohibition of Chemical Weapons (OPCW) at its annual forum in the Hague, said the official report into the attack was “distorting reality”.

The OPCW director general, Fernando Arias, defended the report, telling member countries: “the nature of any thorough inquiry for individuals in a team to express subjective views. While some of these diverse views continue to circulate in some public discussion forums, I would like to reiterate that I stand by the independent, professional conclusion [of the investigation].”

He was supported by the French ambassador, Luis Vassy, who said the Syrian investigation was impartial, professional and rigorous. He added he trusted the OPCW team of experts, and said his country was providing €1m (£854,000) to the investigatory team. Annabel Goldie, the defence minister, said the UK also had full confidence in the hardworking women and men of the OPCW technical secretariat, adding Britain was confident they “would continue to work professionally thoroughly and impartially.”

Shulgin, citing an email from a member of the investigating team dated June 2018 and an earlier document, questioned the conclusion by the OPCW in March 2019 that chlorine was used in Douma.

The leaked email by an investigator described as “Alex” and quoted by WikiLeaks expresses the “gravest concern”, saying the OPCW report “misrepresents the facts”, contains “unintended bias” and simplifies conclusions.

Investigation into alleged use of white phosphorus in Syria Read more

The email says the published OPCW report into the attack changed the language on the levels of chlorine allegedly found compared with what investigators had originally wrote.

It also questions whether or not the chemical came from barrels found at the scene, and whether those barrels had been dropped from the air – which would indicate Assad’s forces – or placed there manually there – which would indicate it was staged by Syrian rebels. The whistleblower said the published report’s conclusions on the barrels deviated significantly from the draft.

The leaks, probably designed to coincide with the start of the OPCW annual conference, were seized on by Russia to construct a claim that the OPCW is gradually being politicised by the west to put Assad on trial for the use of chemical weapons in Syria. Last year, despite Russian objections, the OPCW for the first time was given powers not just to investigate whether an attack had occurred, but identify the parties responsible.

Arias said details of the Douma incident had been passed to the new team set up to name the perpetrators.