The Organisation for the Prohibition of Chemical Weapons (OPCW) said its inspectors had found the toxin used was of “high purity” with an “almost complete absence of impurities”.
Britain said the finding backed its assessment that it was produced in the kind of controlled scientific environment most likely to be found in a state-run laboratory.
The executive summary released by the OPCW does not directly name Novichok – the military grade nerve agent developed by Russia, which the UK has said was used to poison former double agent Sergei Skripal and his daughter, Yulia – or identify its source.
But it states that its analysis of biomedical and environmental samples collected by its inspectors “confirm the findings of the United Kingdom relating to the identity of the toxic chemical that was used in Salisbury”.
It said that the name and structure of the chemical were included in its full classified report made available to member state governments.
The findings were welcomed by Foreign Secretary Boris Johnson, who said they backed Britain’s assertion that only Russia could have carried out the attack in March.
“This is based on testing in four independent, highly reputable laboratories around the world. All returned the same conclusive results,” Mr Johnson said in a statement.
“There can be no doubt what was used and there remains no alternative explanation about who was responsible – only Russia has the means, motive and record.
“We invited the OPCW to test these samples to ensure strict adherence to international chemical weapons protocols. We never doubted the analysis of our scientists at Porton Down.”
He said Britain has called a meeting of the OPCW executive council in The Hague for Wednesday to discuss “next steps”, adding; “The Kremlin must give answers.”
However, Georgy Kalamanov, Russia’s deputy minister of industry and trade, said it was impossible to pinpoint the nerve agent’s origin and reiterated Moscow’s demand for a fresh investigation with Russian involvement.
And Russian foreign ministry spokeswoman Maria Zakharova accused Britain of engaging in “information warfare” over the case.
She said: “The UK authorities are disregarding the standards of international law, diplomatic rules and principles, and elementary human ethics.”
In its report, the OPCW said it had been able to collect blood samples from the Skripals and Detective Sergeant Nick Bailey who was also hospitalised in the incident, plus environmental samples from contaminated “hot spots”.
It also received parts of samples taken by the British authorities which were analysed in OPCW-designated laboratories for “comparative purposes”.