Yulia Skripal, 33, discovered next to her father on bench in Salisbury, is in critical condition.
The second person found unconscious in Salisbury alongside the former Russian spy Sergei Skripal is his 33-year-old daughter.
Yulia Skripal was discovered on a bench next to her father on Sunday and is in a critical condition in hospital. She lives in Russia but was visiting the UK, the BBC reported, adding that relatives had not heard from her for two days.
The apparent poisoning of the pair follows the death of Skripals wife, Liudmila, in 2012. She arrived in Britain with her husband who was swapped in 2010 as part of a spy exchange and had lived with him in Wiltshire.
A certificate recorded the cause of her death on 23 October 2012, aged 59, as disseminated endometrial carcinoma. Skripals son also died recently, aged 43, on a visit to St Petersburg with his girlfriend, the BBC has reported.
Police were continuing on Tuesday to try to establish what the substance was that Skripal, 66, and his daughter were exposed to in Salisbury at the weekend. Both remained in critical condition and in intensive care, police said.
The bench where the pair collapsed unconscious in the Maltings shopping centre next to the river Avon remained cordoned off, as was a Zizzi restaurant, which police said had been shut as a precaution. Wiltshire police revealed a third premises nearby, the Bishops Mill pub, had also been sealed off.
However, Salisbury was still bustling with shoppers and visitors. Police said the advice from Public Health England remained that there did not appear to be any immediate risk to public health.
The pairs sudden and unexplained illness will invite comparisons with the poisoning in 2006 of another Russian spy, Alexander Litvinenko, whose death sparked a major international incident.
The UKs leading counter-terrorism officer said his specialists were supporting the investigation.
The Metropolitan police assistant commissioner Mark Rowley said on Tuesday: Clearly its a very unusual case and the critical thing is to get to the bottom of what has caused this incident as quickly as possible.
As you would expect, the specialist resources that sit within the counter-terrorism network that I coordinate across the country and other partners are working with Wiltshire police to get to the bottom of that as quickly as possible.
He told BBC Radio 4s Today programme: If you look back at other cases like Litvinenko, if necessary we will bring that investigation into the counter-terrorism network. At the moment the key is, though, to get to the bottom of what caused this.
Rowley added: We are doing all the things you would expect us to do. We are speaking to witnesses, we are taking forensic samples at the scene, we are doing toxicology work. That will help us get to an answer. I cant say any more at this stage.
Asked about a series of suspicious Russian-linked deaths in the UK, Rowley said: There are deaths which attract attention. I think we have to remember that Russian exiles are not immortal, they do all die and there can be a tendency for some conspiracy theories. But likewise we have to be alive to the fact of state threats, as illustrated by the Litvinenko case.
A CCTV image of a man and woman walking through an alleyway connecting the Zizzi restaurant and the bench where Skripal was found was believed to be of interest to police.
Police took away an image of two people, shot at 3.47pm on Sunday by a camera at Snap Fitness 24/7, according to the gyms manager, who showed the footage to the Press Association.
Cain Prince, 28, said: Police had a good look at the footage and were interested in these two people. It was the only image they took away. They wanted a list of everyone in the gym between 3pm and 4pm as well. Prince added that police said Skripal was wearing a green coat.
Skripal was one of four Russians exchanged for 10 deep cover sleeper agents planted by Moscow in the US in 2010. For more than five years he has been living quietly though not hidden in a modern red-brick home close to the city centre.
Neighbour James Puttock said Skripal had lived in the area for more than seven years. He was very quiet, Puttock said. If I see him in the street I say hello. Police have been here since Sunday afternoon. Theyre in the house asking questions now.
Puttock, 47, added: He [Skripal] said hello if he walked past, and seemed like a nice chap. When he moved in he invited us all over for a housewarming party I imagine he invited the whole street.
Skripal and his daughter collapsed on Sunday afternoon. A passerby, Freya Church, said: She was leant in on him. It looked like shed passed out. He was doing some strange hand movements, looking up to the sky. I felt anxious, like I should step in but they looked so out of it. They looked like they had been taking something quite strong.
The pair were taken to Salisbury district hospital, where a major incident was delcared on Monday. Public Health England radiation and toxicology experts were called in to help.
Officers in protective suits were seen searching the city centre and Zizzi , on Castle Street in the city centre, was closed on Monday evening in connection with the incident as a precaution.
Skripal is a former Russian army colonel who was convicted of passing the identities of Russian agents working undercover in Europe to MI6 in 2006. He arrived in the UK as part of a high-profile spy swap in 2010.
He was sentenced in August 2006 in Russia to 13 years in jail for spying for Britain after being convicted of high treason in the form of espionage. Russian prosecutors said he had been paid $100,000 (72,000) by MI6 for information he had been supplying since the 1990s when he was a serving officer.
He was flown to the UK as part of an exchange that involved the notorious group of deep cover sleeper agents planted by Russia in the US which included Anna Chapman, a diplomats daughter being taken to Moscow.
It had been assumed that Skripal had been given a new identity, home, and pension. However, Land Registry documents show his house was registered in his real name and was bought for 260,000 with no mortgage on 12 August 2011, just over a year after the spy swap.
Yulia Skripal reported her mothers death to Wiltshire councils register office, telling staff that her father was a retired local government planning officer.
Liudmila, whose maiden name was Koshelnik, and Yulia lived at the same four-bed semi-detached house in Salisbury, according to the certificate.
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