COUNTER-TERRORISM detectives have urged anyone in Salisbury city centre on Sunday to come forward to help with the "missing pieces" in the probe into how a Russian double agent and his daughter fell critically ill.

Sergei Skripal, 66, and 33-year-old Yulia remain fighting for their lives in hospital after being exposed to a mystery substance which has triggered a diplomatic row and prompted crisis talks in Whitehall.

As the high-profile investigation enters a fourth day, counter-terror officers have extended the cordon further after the pair were found unconscious in the city shortly after 4pm on Sunday.

Assistant Commissioner Mark Rowley, the head of counter-terrorism policing, stressed to people in the city, particularly those who visited the Bishop's Mill pub or Zizzi restaurant, that any details could help investigators.

He said: "We would like to hear from anybody who visited the area close to the Maltings shopping centre where these two people were taken ill on Sunday afternoon, and may have seen something that could assist the investigation.

"The two people taken ill were in Salisbury centre from around 1.30pm. Did you see anything out of the ordinary? It may be that at the time, nothing appeared out of place or untoward but with what you now know, you remember something that might be of significance.

"Your memory of that afternoon and your movements alone could help us with missing pieces of the investigation. The weather was poor that day so there were not as many people out and about. Every statement we can take is important."

Scotland Yard said detectives were "keeping an open mind as to what happened", and that the situation had not been declared a terrorist incident - adding that it would not "provide a running commentary" on the investigation.

Mr Skripal was found along with his daughter on a bench in The Maltings in Salisbury after police were called by a concerned member of the public at around 4.15pm on Sunday.

The pair did not have any visible injuries and were taken to Salisbury District Hospital, where they are being treated in intensive care for "suspected exposure to an unknown substance".

Officers "secured" a number of scenes - including the Zizzi restaurant on Castle Street and the Bishop's Mill pub in The Maltings - as well as setting up a cordon at Solstice Park in Amesbury.

Scotland Yard said: "This is linked to the investigation and is a precautionary measure."

At least two people left a contamination tent inside the cordon wearing protective suits and gas masks on Tuesday night.