The nerve agent that poisoned Sergei Skripal, his daughter Yulia and now a couple in Amesbury is one of the most deadly chemical weapons ever developed and was produced in secret by the former Soviet Union.

Variants of the nerve agent Novichok are reported to be up to eight times more effective than VX nerve gas, that was deployed in the assassination of Kim Jong-nam, the estranged half brother of the North Korean leader Kim Jong-un in February 2017.

One of Novichok’s developers - a Russian scientist Vil Mirzayanov who fled to the US after turning whistle-blower - has said one of the variants had no known cure.

Scientists at the defence laboratory at Porton Down, just seven miles from Salisbury, identified the use of Novichok as the weapon used in the attempted murder four months ago of Colonel Skripal, 66, a Russian double agent who sold secrets to MI6, and his 33-year-old daughter.

On Wednesday, police said tests carried out at the Porton Down had established that Charles Rowley, 45, and his girlfriend, Dawn Sturgess, 44, had also been exposed to Novichok.