A third Russian agent implicated in the Salisbury nerve agent attack aborted his planned exit from the UK, raising the prospect he remained in the country.

The Russian military intelligence officer - using the false name Sergey Fedotov - travelled to the UK on the same day as two hitmen who carried out the attempted assassination of Sergei Skripal.

Fedotov was then booked on to the same flight back to Moscow with the assassins. But it is now understood, according to sources, that Fedotov checked himself and his baggage off the plane before departure.

It is not clear why but it raises the prospect that Fedotov stayed in the UK, at least in the days after the deployment of military-grade nerve agent in Salisbury in March last year.

A source told The Telegraph: “It is not clear why Fedotov did not board the flight. But at the last minute he checked himself and his bags off it. He could still have been running around Britain.”