Government scientists are analysing the substance at the centre of the Russian spy attack, amid fears it was a deadly nerve agent used in previous high profile political assassinations.

Experts from the government's chemical defence laboratory at Porton Down, just six miles from where Sergei Skripal was targeted in Salisbury, were understood to be urgently trying to identify the chemical.

The former Russian spy and his 33-year-old daughter Yulia, who was with him at the time, were rushed to Salisbury District Hospital, where a major incident was declared, amid fears the mysterious substance could result in further casualties.

One theory being explored was that the substance could be the deadly nerve agent, VX, which was used last year in the murder of Kim Jong-nam – the estranged half brother of the North Korean leader Kim Jong-un.

The chemical was first developed by the British firm, ICI, in the 1950s, but was put to deadly use by Saddam Hussein in an attack against the Kurds in 1988.