The Russian former double-agent who collapsed in a British town alongside his daughter was targeted with a nerve agent, while a responding policeman has also been hospitalised, police said Wednesday.
"This is being treated as a major incident involving attempted murder by administration of a nerve agent", Metropolitan Police assistant commissioner Mark Rowley told reporters.
Scientific tests by government experts have identified the specific nerve agent used, "which will help identify the source" police said, but Rowley declined to reveal the exact substance.
Sergei Skripal, 66, who moved to Britain in a 2010 spy swap, is in a critical condition in the hospital along with his daughter Yulia after they collapsed on a bench outside a shopping centre in the southwestern English city of Salisbury on Sunday.
"Sadly, in addition, a police officer, who was one of the first to attend the scene is now also in a serious condition in hospital," said Rowley.
Britain's Sky News, quoting sources, reported late Wednesday that all three victims are in a coma.
Other emergency services personnel who treated the pair required medical treatment at the time but have not been admitted to hospital. Interior minister Amber Rudd called for "cool heads" over the poisoning, which is already being linked with Russia by British politicians and the media.
She is expected to make a statement in parliament on Thursday.
Police say they are keeping an open mind about what happened, but Foreign Secretary Boris Johnson has alluded to Russia.
He noted the "echoes" with the 2006 poisoning in London of former Russian spy and Kremlin critic Alexander Litvinenko, which Britain has blamed on Russia.
Moscow accused British politicians and journalists of whipping up anti-Russian sentiment, with Kremlin foreign ministry spokeswoman Maria Zakharova telling reporters the story "was straight away used to boost an anti-Russian campaign in the media".
Zakharova earlier said Johnson`s comments were "wild".
Meanwhile, hundreds of counter-terrorism detectives are working "around the clock" to create a timeline of the victims' movements, with "many hours" of CCTV under review, police said.
Investigators believe Skripal and his 33-year-old daughter were in Salisbury city centre for several hours before they were found slumped on a bench.
They reportedly had lunch at a pizza restaurant, Zizzi, and visited a pub in Salisbury before being discovered outside the shopping centre, where onlookers said they appeared "out of it".
An anonymous witness who was in the pub, which has been closed by police, told the BBC on Wednesday that Skripal was there behaving erratically and at one point shouting loudly.
Rowley appealed for information from those in Salisbury on Sunday.
"Your memory of that afternoon and your movements alone could help us with missing pieces of the investigation," he said.
The Times newspaper reported police are probing whether Skripal's daughter, who arrived in Britain from Moscow last week with "gifts from friends", may have brought the nerve agent into the country.
The paper previously said investigators would also examine the 2012 death of Skripal's wife from cancer, and that of his 44-year-old son last year in St Petersburg, reportedly from liver problems. Prime Minister Theresa May was updated on the case at a meeting of her national security council on Tuesday but has declined to publicly comment on the ongoing investigation.
However, she confirmed the government might consider a boycott by British officials and dignitaries of the 2018 football World Cup in Russia if it were found to have been involved.
"Depending on what comes out in relation to the investigation... it might be appropriate for the government to look at whether ministers and other dignitaries should attend the World Cup in Russia," she said Wednesday.
The possible boycott -- which would not include players -- was first raised by Johnson on Tuesday when he told MPs that he was not pointing fingers for Skripal's collapse but made several references to Russia.
He warned Britain would respond "appropriately and robustly" if a government was found responsible.
Multiple British media outlets reported Wednesday that Prince William would now not attend this summer's football World Cup in Russia, citing royal sources.
The prince has attended recent tournaments in his role as president of the English Football Association.
Kensington Palace, his residence, did not respond to a request for comment.
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 United States in 2010.