A forensic tent covers the bench where Sergei Skripal and his daughter Yulia were found. Picture: REUTERS
A forensic tent covers the bench where Sergei Skripal and his daughter Yulia were found. Picture: REUTERS

London — Britain has vowed to act "without hesitation" if a state is found responsible for a nerve agent attack on a former Russian double agent.

Police have said 21 people in total had received medical treatment following the incident.

Sergei Skripal and his daughter Yulia remain unconscious in a critical but stable condition following the attack on Sunday in the sleepy southwestern English city of Salisbury.

Sergeant Nick Bailey, one of first officers to find the pair slumped on a bench outside a shopping centre and who was later hospitalised in intensive care, was sitting up and talking on Thursday, according to police.

"There’s been around 21 people (treated)," said Kier Pritchard, chief constable for Wiltshire Police, noting that included "multiple officers".

"A number of those have been through the hospital treatment process, they’re having blood tests, they’re having treatment in terms of support and advice," he said.

Pritchard said he had spoken to Bailey and his wife in hospital, and "he’s well, he’s sat up" but was "very anxious".

Authorities are racing to identify the nerve agent used against the 66-year-old Skripal, who came to Britain in a spy swap in 2010, as politicians warned it showed the hallmarks of an attack by Russia.

Interior minister Amber Rudd told MPs that the "brazen and reckless" attack was "attempted murder in the most cruel and public way" but declined to single any perpetrator out.

Noting it was "highly likely" that the officer, Bailey, was exposed to the same nerve agent as the Russian pair, she said Britain "will act without hesitation as the facts become clearer".

The Russian embassy in London, which earlier in the week criticised Foreign Secretary Boris Johnson for blaming Moscow, welcomed Rudd’s "responsible, practical approach".

Prime Minister Theresa May said that "if action needs to be taken then the government will do that", but urged "time and space" for the police to conduct their investigation.

National counterterrorism police have taken over the case, and on Wednesday confirmed that a nerve agent was used, saying they were treating the attack as attempted murder.

"Scientific tests by government experts have identified the specific nerve agent used which will help identify the source," they said.

Police have cordoned off the bench where the pair were found, as well as an Italian restaurant and a pub they visited before their collapse.

They also sealed off the grave of Skripal’s wife, Liudmila, who died in 2012 from cancer, as well as the memorial stone of his son, Alexander, who was cremated last year after reportedly dying of liver problems.

As officers in biohazard suits worked inside secure tents, residents expressed shock at the attack in their midst.

"You could be walking by and you could be involved in it, you know, it’s quite shocking and worrying," Salisbury local Jackie Tothill said.

The Times reported that police were probing whether Skripal’s daughter, who arrived in Britain from Moscow last week, may have inadvertently brought in the nerve agent as a gift.

Johnson has noted the "echoes" with the 2006 poisoning in London of former Russian spy and Kremlin critic Alexander Litvinenko, which Britain has blamed on Russia.

Defence Secretary Gavin Williamson said on Thursday that Russia was "becoming an ever-greater threat". Fellow Conservative MP Nick Boles tweeted: "I do not see how we can maintain diplomatic relations with a country that tries to murder people on British soil".

Moscow accused British politicians and journalists of whipping up anti-Russian sentiment.

Kremlin foreign ministry spokeswoman Maria Zakharova told reporters the story "was straight away used to boost an anti-Russian campaign in the media".

The embassy said on Thursday that when other Russians had died in Britain in suspicious circumstances, the authorities had never released the findings, adding: "Same happening now."

It cited Boris Berezovsky, a friend of Litvinenko found hanging in his bathroom in 2013, and Alexander Perepilichny, a businessman found dead in front of his home. No cause of death was determined in either case.

Skripal was a former colonel in Russian military intelligence who was jailed in his country for betraying agents to Britain’s MI6 secret service.

He was pardoned before being flown to Britain as part of a high-profile spy swap involving Russia and the US in 2010.