The head of a research laboratory in Salisbury has refuted Russian claims that the nerve agent used on a Russian ex-spy could have originated there.
Gary Aitkenhead, Chief Executive of Porton Down Defence Science and Technology Laboratory, said they have the "highest levels of controls of security".
Russia said that the proximity of the lab to the incident could be seen as suspicious.
However, Aitkenhead said such claims were "frustrating" and that he had "complete confidence" that nothing could have gone from the lab to the "wider world".
Russian ex-spy Sergei Skripal and his daughter were poisoned in Salisbury with a military-grade nerve agent, British authorities said.
Britain is convinced Moscow was behind the nerve-agent attack.