Scientists from Porton Down have not been able to establish where the novichok nerve agent used to poison Sergei and Yulia Skripal was made.
Gary Aitkenhead, chief executive of the Defence Science and Technology Laboratory (DSTL) at Porton Down, told Sky News they had not been able to prove it was made in Russia.
He said: “We were able to identify it as novichok, to identify that it was military-grade nerve agent.”
Mr Aitkenhead would not comment on whether Porton Down keeps novichok.
Porton Down’s boss would not comment on whether the lab had developed or keeps stocks of novichok, but dismissed suggestions the agent used to poison the Skripals had come from Porton Down. “There is no way anything like that could have come from us or left the four walls of our facility,” said Mr Aitkenhead.