The former MI6 spy who was poisoned in a botched assassination attempt by Russian agents in Salisbury has suffered a deterioration in his health and is being treated by doctors, The Sunday Times can reveal.

Sergei Skripal, 67, who along with his daughter, Yulia, 33, was poisoned with the nerve agent novichok last March, is under medical supervision amid concerns that he is unlikely ever to recover fully from the chemical attack which assaults the nervous system.

The former colonel in the GRU, Russia’s military intelligence agency, and his daughter were in a critical condition when they were admitted to Salisbury District Hospital after the attack.

The pair staged a miraculous recovery — Yulia was discharged in April last year and her father was released