Yulia Skripal, 33, was found slumped next to her father, the former spy Sergei Skripal, at a shopping centre in Salisbury

The suspected poisoning of a former Kremlin double agent and his daughter is being treated as an assassination attempt linked to Russia, Whitehall sources said last night.

Sergei Skripal, 66, and his daughter Yulia, 33, remained critically ill in hospital yesterday after being exposed to an unknown substance and collapsing at a shopping centre in Salisbury on Sunday. Counterterrorism detectives from Scotland Yard have taken over the inquiry from Wiltshire police and Amber Rudd, the home secretary, will chair a meeting of the Cobra crisis committee today.

Sergei Skripal was detained in Russia before arriving in Britain as part of a spy swap deal REUTERS

Sources said that early indications pointed to a state-sponsored assassination attempt and it was being treated as such by police and MI5. Alternative theories — such as a rival faction wanting to frame Russia and undermine President