A Russia-led campaign that claimed the UN weapons watchdog had manipulated evidence of a Syrian government chemical weapons attack has been dealt a blow by an official inquiry showing that two former employees hailed as whistleblowers had little direct access to the evidence and inflated their role.

The independent inquiry commissioned by the Organisation for the Prohibition of Chemical Weapons (OPCW) shows that one of the two had never been on the team investigating the April 2018 attack in Douma and the other was only on the team for a brief period.

Both individuals, referred to in the report as Inspector A and B, may face legal action.

More than 40 people were killed on 7 April 2018 in the town of Douma on the outskirts of Damascus. The town was at the time held by rebels but besieged by pro-government forces. Civilians claimed they were the victims of a chemical weapon attack.

The assault prompted reprisal missile strikes on Syrian government targets by the US, Britain and France a week later, one of the few direct strikes on Syrian government assets in the nine-year civil war.

Russia immediately launched a campaign, including bringing witnesses from Syria to the OPCW headquarters in the Hague to challenge the claim that chemical weapons were used.

The charge that Russia permits, or covers up the Syrian president, Bashir al-Assad’s use of chemical weapons on Syrian civilians, is one of the most bitter flashpoints between Russia and the west.

Following the US strike, the OPCW set up a fact-finding mission to decide whether chemical weapons had been used, but not to attribute responsibility. It ruled in March 2019 that a banned toxic chemical containing chlorine was likely to have been used in Douma. The fact that chemical weapons were delivered through airstrikes effectively meant the OPCW believed the Syrian air force was responsible.

Internal OPCW reports questioning whether chemical weapons had been used leaked last May, raising questions about manipulation of the OPCW by the west.

But an OPCW inquiry into those leaks published on Thursday found the authors of the internal reports only had a minor supporting role in the Douma team.

The OPCW chief Fernando Arias told OPCW states in comments published on Thursday that the two individuals were “not whistleblowers”. He said: “They are individuals who could not accept that their views were not backed by evidence.” He added that the two men breached their obligations to the organisation, saying their behaviour was even more egregious since they had manifestly incomplete information on the investigation.

The official inquiry said: “Inspector A did not have access to all the documents, witness interviews, laboratory tests and analyses by independent experts.” It said he never wrote an official OPCW report, and only wrote a personal document created with incomplete information.

It added: “Inspector B was by contrast on the fact-finding mission and did travel to Damascus in April, but never left the command post because he had not completed the necessary training required to be deployed on-site in Douma. He left the OPCW in August, but continued to approach staff members in an effort to have continued access to and influence over the Douma incident. The majority of the fact-finding team’s work was carried out after he left the organisation.”

The two whistleblowers declined to take part in the investigation that met instead with 29 witnesses between July 2019 and this month.

Some have criticised the OPCW for being so slow to challenge the validity of the leaks, so ceding the ground to those who believe the west and its intelligence agencies fabricate evidence of chemical weapons use to discredit Assad.

Tobias Schneider, a research fellow at Berlin’s Global Public Policy Institute, said: “I thought the organisation was slow and reactive in their initial response to the leaks, ceding the field and allowing the narrative to spin out of control. This is not surprising. The OPCW secretariat is very careful to avoid anything that might be viewed as ‘politicising’ what they see as their ‘technical’ work. Obviously, not all signatory states see eye to eye on this issue. So they [the secretariat] fell back on the independent investigation as a procedural tool.

“The final report of that investigation, as well as the remarks of the director general himself, I think were extraordinarily and unexpectedly forceful and direct.

“My sense is that the director general and the secretariat realised that this disinformation campaign represented a direct assault not only on the organisation (and thus the convention it is meant to uphold) but also on the professionalism and reputations of those that serve in their ranks, oftentimes in extremely difficult circumstances.

“I am glad they chose not to not mince words and unequivocally dismissed these faux leaks.”