Police and intelligence agencies have identified key suspects in the attempted assassination of Sergei Skripal and his daughter Yulia, The Telegraph understands.

Counter-terrorism police are now trying to build a case against “persons of interest”, who are believed to be back in Russia.

The Telegraph has been told that the criminal inquiry, expected to take many more months, is understood to have made a major breakthrough in identifying key people over the nerve agent attack.

It is thought that a search of flight manifests in and out of the UK has yielded specific names in the hunt for the Skripals’ would-be assassins. Police have also drawn on extensive CCTV footage in Salisbury and have also trawled car number plate recognition cameras.

But counter-terror police will hit a diplomatic brick wall in trying to interview - let alone prosecute - the suspects.