“Should evidence emerge that implies state responsibility, then Her Majesty’s government will respond appropriately and robustly,” said Mr. Johnson, who confirmed the identities of the victims. “Though I am not now pointing fingers, I say to governments around the world that no attempt to take innocent life on U.K. soil will go unsanctioned and unpunished.”
The comment drew a rebuke from the spokeswoman for the Russian Foreign Ministry, Maria Zakharova. “This is simply ignorance,” she said, the Interfax news agency reported.
Ms. Zakharova said she was bewildered that “the man, who works in foreign policy and has nothing to do with law enforcement, was making such claims.” She said, “We presume that law enforcement and security agencies first will get at least some real evidence and then share it.”
Mr. Skripal and his daughter remained in critical condition on Tuesday at Salisbury District Hospital, about 85 miles southwest of London. The police said they had suffered “exposure to an unknown substance.”
With its echoes of stranger-than-fiction plots from the Cold War and earlier episodes from the Putin era, the case threatens to worsen the already tense relations between the West and a Russian government that has annexed Crimea, destabilized eastern Ukraine and propped up the government of Bashar al-Assad in Syria, all while being accused of disrupting elections and sowing discord within Western democracies.
“This is a form of soft war that Russia is now waging against the West,” said Tom Tugendhat, chairman of the Foreign Affairs Committee in the British Parliament.
British officials have accused the Kremlin of only one assassination on British soil in recent years, but the Russian government has been suspected of being behind numerous other mysterious deaths in Britain and elsewhere.
In Mr. Litvinenko’s case, the weapon is believed to have been a poisoned teapot later found to contain polonium 210, a radioactive isotope; his death was slow and agonizing. In 1978, when Georgi Markov, a Bulgarian dissident, was killed on Waterloo Bridge in London, the weapon was an umbrella tipped with a pellet of ricin.
The Skripal family has had its own suspicions about the Russian government, the BBC reported. Mr. Skripal’s brother died two years ago, and just last year, the family said, his son succumbed to liver failure while on vacation in St. Petersburg, Russia. Mr. Skripal’s wife, Lyudmila, died of cancer in 2012.
Mr. Tugendhat said that Britain should consider revoking the broadcast license of RT, the Kremlin-funded channel formerly called Russia Today. “I see absolutely no reason why we should allow information warfare to be carried out on U.K. soil by hostile agents,” he said.
Citing the unusual circumstances, the Metropolitan Police Service put Britain’s counterterrorism police in charge of the case on Tuesday, though it has not yet been determined whether terrorism was involved.
“We’re speaking to witnesses, we’re taking forensic samples at the scene, we’re doing toxicology work,” Mark Rowley, who leads the counterterrorism force, told BBC radio.
The idyllic setting of the episode, deep in the English countryside, a few hundred meters from one of Britain’s oldest cathedrals, struck many as bizarre. In a town best known for its quaint streets and postcard-perfect cottages, as well as its proximity to Stonehenge, the world-renowned neolithic monument beyond the hills nearby, residents were stunned by their sudden proximity to international intrigue.
“We’re as surprised as anybody at our elevation to the national stage,” said John Walsh, a member of Mr. Skripal’s neighborhood council. He said he had visited residents near Mr. Skripal’s home a few days ago, “sorting out a problem with a hedge — which is more my level of activity, rather than high espionage.”
Police specialists wearing hazardous material suits cordoned off part of the shopping district where the Skripals were found, while officers from multiple law enforcement agencies combed the area for evidence.
“It’s the talk of the town,” said Frogg Moody, another councilor from Mr. Skripal’s district. “It’s absolutely taken Salisbury by storm.”
On Mr. Skripal’s quiet street in the west of the town, his neighbors seemed shocked by both the sudden carousel of journalists and by the news that they had been living next to a former Russian spy.
“He was just an ordinary person,” said James Puttock, a 47-year-old scaffolder who lives four doors down from Mr. Skripal. “I didn’t think he was a Russian spy.”
“How do you even know?” he mused. “Do I look like a Russian spy?”
In 2006, Mr. Skripal, a former colonel in the Russian military intelligence agency G.R.U., was convicted in a Russian court of being a double agent, and secretly passing classified information to British intelligence. When he was arrested, the Federal Security Service in Russia said he had started spying for Britain in 1995, when he was stationed overseas, and continued to do so after retiring from the military in 1999.
In 2010, Mr. Skripal was released from prison and sent to Britain as part of a spy exchange with Western agencies. That deal involved Anna Chapman, a Russian sleeper agent based in the United States who has maintained a decidedly less low-profile life than Mr. Skripal has. Ms. Chapman went on to a career in television, has developed her own clothing line and just this week was photographed posing on a beach in Thailand.
Yulia Skripal’s Facebook page says that she graduated in 2008 from the Russian State University for the Humanities in Moscow, lived in several places in southern England beginning in 2010 and by 2016 had moved back to Moscow. It is not clear why she was in Salisbury on Sunday, or how long she had been there.
Marina Litvinenko, the wife of the Russian spy killed in London in 2006, told the BBC that the plight of Mr. Skripal was “like déjà vu,” and suggested that Russia was behind his apparent poisoning.
The Russian government said it had no information about the incident, and Dmitri S. Peskov, a Kremlin spokesman, said accusations of Kremlin involvement “weren’t long in coming.”
“You know how he ended up in the West, what actions and decisions led him there,” Mr. Peskov said.
Mr. Rowley, the British counterterrorism official, cautioned against jumping to conclusions. “I think we have to remember that Russian exiles aren’t immortal,” he said. “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.”
On Tuesday afternoon, amid reports that some of the first emergency workers to tend to Mr. Skripal and his daughter were later treated for wheezing and eye problems, the bench where the pair was found was still cordoned off. It was obscured by a white-and-yellow evidence tent.
An earlier version of a picture caption with this article misstated when a Russian intelligence officer and his daughter were found unconscious. It was Sunday, not Monday.