Sergei Skripal and his daughter may have been exposed to the nerve agent used in their attempted assassination through the Russian spy’s car ventilation system, intelligence sources have told a US television channel.

ABC News said sources had told it that intelligence officials “now have a clearer picture of just how the attack was conducted”.

Agencies reportedly now believe the toxin - identified as a fourth generation nerve agent called Novichok - was used in a “dust-like powdered form” and that it circulated through the vents of Colonel Skripal’s BMW.

ABC News, citing three intelligence officials, said that the Novichok agent had been delivered in a powdered form.

“It is a Cold War substance, something they [Russia] claimed never to have,” an intelligence official told ABC News.