The head of the UK's chemical weapons facility at Porton Down says the substance used in last month's attack in Salisbury was the nerve agent known as Novichok.
Three weeks on from the attack, Sergei Skripal remains in a critical but stable condition. His daughter Yulia is reported to be getting better.
Russia has denied it was behind the attack in Salisbury on March 4.
Speaking to Forces News, Gary Aitkenhead said his team concluded that this was a military-grade nerve agent and it was highly likely that only a state or someone with very advanced facilities would be able to deliver something like this.
"We've never seen anything like this on UK soil before"
But Mr Aitkenhead has also said his scientists haven't been able to identify the exact source of the substance. His comments came a day ahead of a meeting in The Hague of the executive council of the Organisation for the Prohibition of Chemical Weapons to discuss the Salisbury attack.
The Prime Minister Theresa May insists it's almost certain Russia was involved in poisoning the former spy Sergei Skripal and his daughter Yulia.
Mr Aitkenhead told Sky News it was not Porton Down's role to work out the origin of the nerve agent. He also denied claims by Russia that the substance could have come from Porton Down itself.