Up to 100g of Novichok nerve agent may have been used to poison a Russian ex-spy and his daughter in Salisbury, the chemical weapons watchdog has said.
The head of the Organisation for the Prohibition of Chemical Weapons (OPCW) told the New York Times the amount used implied it was created as a weapon.
Ahmet Uzumcu said the samples collected by the OPCW were also of “high purity”.
Sergei and Yulia Skripal were found unconscious on a bench on 4 March after being exposed to the nerve agent.
The UK accuses Russia of being behind the attack, but Russia strongly denies this.
Speaking to the newspaper, Mr Uzumcu explained that the amount of Novichok thought to have been used was significantly more than would be needed for research purposes.
He said only “five to 10g” would be required in such cases, but that inspection of various sites in Salisbury indicated “50, 100g or so” had been used.
That amount, he said, “goes beyond research activities for protection”.
Mr Uzumcu also said the nerve agent was in liquid form and could have been applied with an aerosol spray or, “if you take the necessary measures, you could use it as a liquid”.
“One thing, perhaps, which is important to note is that the nerve agent seems to be very persistent,” he added.
“It’s not affected by weather conditions. That explains, actually, that they were able to identify it after a considerable time lapse. We understand it was also of high purity.”
Mr Skripal, 66, remains in hospital but Ms Skripal, 33, was discharged on 9 April and taken to a secure location.
Wiltshire Police Det Sgt Nick Bailey, who attended the scene, was also treated in hospital after being exposed to the chemical, but has since been discharged.
A clean-up operation is under way in the Wiltshire city, although no time-frame has been given for its completion.
The UK has blamed Moscow for the attack, expelling 23 Russian diplomats and their families.
Twenty-nine countries also expelled 145 Russian officials in solidarity with the UK – and Nato ordered 10 Russians out of its mission in Belgium.
Moscow has denied any involvement and has responded by expelling British and US diplomats, as well as several from other countries.
It has also closed the British Council in Russia and the British Consulate in St Petersburg.