Scientists at the UK’s Porton Down defence laboratory have not been able to determine where the nerve agent used in the Salisbury spy attack was made, the boss of the facility has revealed.

Gary Aitkenhead, the chief executive of the Defence Science and Technology Laboratory (DSTL) at Porton Down, said that chemical weapons experts had “not verified the precise source” of the material but making the substance was "probably only within the capabilities of a state actor".

However, he said that “it is not our job” to determine precisely where the nerve agent, identified as belonging to the Novichok family, was manufactured but he explained the work done at Porton Down formed part of the Government's wider intelligence picture.

Mr Aitkenhead also poured cold water on Kremlin suggestions that the material used to poison the former double agent Sergei Skripal and his daughter Yulia may have come from Porton Down which is located nearby to Salisbury.

Mr Aitkenhead told Sky News: “We were able to identify it as Novichok, to identify that it was a military-grade nerve agent.