As much as 100 grams of nerve agent may have been used in the poisoning of former Russian spy Sergei Skripal and his daughter Yulia.
Ahmet Uzumcu, director general of the Organisation for the Prohibition of Chemical Weapons (OPCW), told the New York Times the relatively large quantities of novichok used suggested it had been created as a weapon rather than for research purposes.
The Skripals were left fighting for their lives in hospital after being exposed to a liquid form of the chemical in the Wiltshire city of Salisbury on 4 March. Both are now recovering from the attack.
Mr Uzumcu said samples collected suggested the nerve agent used to poison Mr Skripal, 66, and his 33-year-old daughter were of "high purity".
He said: "For research activities or protection you would need, for instance, five to 10 grams or so, but even in Salisbury it looks like they may have used more than that.
"Without knowing the exact quantity, I am told it may be 50, 100 grams or so, which goes beyond research activities for protection.
"It's not affected by weather conditions. That explains, actually, that they were able to identify it after a considerable time lapse."
Britain has accused Russia of carrying out an attempted assassination of former double agent Mr Skripal, an allegation Moscow has routinely denied.
The Russian ambassador to the UK, Alexander Yakovenko, has previously suggested the Skripals were injected by British authorities with a nerve agent produced at the government’s Porton Down laboratory.
Russian foreign minister Sergei Lavrov has also claimed the agent used in the attack was in fact a toxin possessed by the UK and US.
The OPCW’s independent report on samples of the substance collected from the scene did not refer to novichok by name.
It did however say its scientists had "confirmed the findings of the United Kingdom relating to the identity of the toxic chemical".
The affair has exacerbated the already strained relations between Russia and the West, leading to the expulsion of dozens of diplomats by both sides.
Karen Pierce, the UK's representative to the United Nations, told a meeting last month there was "no plausible alternative explanation than Russian state responsibility for what happened in Salisbury".
"Russia has a proven record of conducting state-sponsored assassinations including on the territory of the United Kingdom," she said.
"The independent inquiry into the death of Alexander Litvinenko concluded in January 2016 that he was deliberately poisoned with polonium, that the FSB had directed the operation and that President Putin probably approved it."
Meanwhile, government scientists have warned potentially toxic levels of the nerve agent may still be present in "hot spots" around Salisbury.
Residents of the city have criticised authorities for the slow pace at which safety information has been provided to the public, accusing certain bodies of sending "mixed messages" as to potential risks.
Wiltshire Council said new barriers set up around The Maltings shopping centre, The Mill pub, Zizzi restaurant and Riverside House are expected to be in place for several months while a multi-million pound clean-up operation takes place.
It said the task would be carried out by 200 specialist military personnel, insisting all areas outside the cordons were safe.
Additional reporting by PA
