Sergei, 66, and Yulia Skripal, 33, are in a critical condition after they were 'specifically targeted' with a deadly substance in the middle of Salisbury town centre in Wiltshire.
It is also been revealed that a police officer who was among the first on the scene and found the pair slumped on a bench is in a 'very serious' condition in hospital.
All three are believed to be in comas in intensive care following exposure to the nerve agent on Sunday.
Last night a source told The Times that the feeling was Mr Skripal 'was not going to make it', while the prognosis for his daughter was slightly better and the policeman was less seriously ill.
It also emerged last night that Whitehall officials were drawing up a list of Russian officials they could expel from the country if it emerged Moscow was behind the assassination attempt.
Nerve agents are highly toxic chemicals that attack the nervous system and shut down bodily functions.
There were also claims in The Times today that investigators are probing whether Yulia could have unwittingly brought the toxin with her from Russia.
A source said she was thought to have brought a 'gift from friends' from Russia which might have been opened when the pair were out in Salisbury on Sunday.
Speaking outside Scotland Yard on Wednesday, head of counter terrorism policing, Mark Rowley revealed government experts have identified the specific nerve agent that was used, but would not be making that information public.