Russia has denied any responsibility.
In an interview on Monday, Secretary of State Rex. W. Tillerson, expressed astonishment at the use in a public space of a substance like the nerve agent. “It’s almost beyond comprehension that a state, an organized state, would do something like that,” he said. “A nonstate actor, I could understand. A state actor I cannot understand why anyone would take such an action.”
Mrs. May said that her the government had summoned the Russian ambassador to demand an explanation, and that Britain expected a response from Russia by the end of the day on Tuesday. “Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian state against the United Kingdom, and I will come back to this House and set out the full range of measures we will take in response,” she said.
“We shall not tolerate such a brazen act to murder innocent civilians on our soil,” the prime minister said.
The relationship between Russia and Britain under Prime Minister May has been punctuated by repeated confrontation, over the annexation of Crimea and Russian interference in elections, among other issues.
But Britain has held back from aggressive retaliatory measures. Expelling Russian spies, for example, would mean a cutoff in Britain’s own flow of information from Moscow if Russia retaliated. Restricting visas would hurt Russian businessmen, officials and dissidents who have made Britain their home.
Earlier on Monday, before Mr. Tillerson spoke, the White House took a different approach, declining to point a finger at Russia.
Sarah Huckabee Sanders, the White House press secretary, said at her daily briefing: “The use of a highly lethal nerve agent against U.K. citizens on U.K. soil is an outrage. The attack was reckless, indiscriminate and irresponsible. We offer the fullest condemnation.”
But Ms. Sanders brushed off several questions about whether the White House shared Britain’s view that Russia was responsible. “Right now we are standing with our U.K. ally,” she said. “I think they’re still working through even some of the details of that and we’re going to continue to work with the U.K.”
Moscow has insisted that it played no role in the attack, and did so again on Monday.
“This is a circus show in the British Parliament,” the Russian Foreign Ministry spokeswoman, Maria Zakharova, told journalists in Moscow, according to the Interfax news agency.
Vladimir Dzhabarov, first deputy head of the Federation Council’s foreign affairs committee, was equally dismissive. Whatever Mr. Skripal may have once done, he said, he posed no threat to Russia now.
“This already is not our issue,” Mr. Dzhabarov told Interfax. “He had access neither to our secrets nor facilities. He was of no use to us, to Russia in general.”
Still, amid denials last week by Sergey V. Lavrov, Russia’s foreign minister, an anchor on Russia’s state-controlled news broadcast struck a different note, warning Russians not to betray their country. If they do, he said, “Don’t choose Britain as a place to live.”
In her address to Parliament, Mrs. May said the nerve agent was part of a group known as Novichok — the Russian term for “newcomer.” The chemical was produced by the Soviet Union in the 1970s and 1980s, and, at the time, it was believed to be far more lethal than anything in the United States arsenal.
After the breakup of the Soviet Union, Vil Mirzayanov, a chemist who helped develop the agent, said that Soviet laboratories had developed enough of the substance to kill several hundred thousand people.
Dispersed in a powder, Novichok nerve agents blocked the breakdown of a neurotransmitter controlling muscular contractions, leading to respiratory and cardiac arrest, Mr. Mirzayanov told investigators at the time.
The use of a nerve agent drew the attention of the Organization for the Prohibition of Chemical Weapons, the group that polices the global treaty banning them. The group, based in The Hague, called it “a source of great concern.”
Almost two dozen people in Salisbury, including emergency workers, were given medical treatment related to the attack, and one police officer is still hospitalized.
Over the past week, chemical weapons experts fanned out through the sleepy cathedral city of Salisbury, and residents who may have been near Mr. Skripal and his daughter were told to wash their clothing and carefully wipe off other articles. Politicians have urged the government to respond.
“What it says to Russians living in the U.K. or those thinking of leaving the country is: Disloyalty is always punishable, you will never be free of us and you will never be safe, wherever you live,” John Lough and James Sherr, Russia specialists at the British think tank Chatham House, wrote. “What it says to the British government is: We believe you are weak, we have no respect for you.”
Mr. Skripal is one of several opponents of President Vladimir V. Putin’s government, in Britain and elsewhere, who have been the victims of murder or attempted murder. Western intelligence officials say that the Kremlin has frequently had its foes killed. The most notorious case involved another former Russian agent, Alexander V. Litvinenko, who was fatally poisoned in London in 2006 with a radioactive element, an assassination that a British inquiry later concluded was probably approved personally by Mr. Putin.
The British government has, however, been accused of dragging its feet in investigating previous suspicious deaths.
On Tuesday, Yvette Cooper, a lawmaker with the opposition Labour Party, submitted a letter to Britain’s home secretary demanding a review of 14 deaths which “have not been treated as suspicious by the U.K. police but have — reportedly — been identified by United States intelligence sources as potentially connected to the Russian state.”
But with the intense attention focused on the poisoning of Mr. Skripal, 66, and his daughter, 33, the government response has been swifter.
Officials from across the British political spectrum have called for a wide range of retaliatory measures against Russia, including the expulsion of diplomats, new economic sanctions, tighter controls on wealthy Russians entering Britain, and the revocation of the broadcast license of RT, the Kremlin-controlled broadcaster.
Britain must ensure that Russia’s oligarchs “realize that they can’t spend their wealth in London, that they can’t enjoy the luxuries of Harrods and whatever else, and that we’re absolutely firm in making sure that they feel the pain of being denied entry into the West,” Tom Tugendhat, chairman of the Foreign Affairs Committee in the House of Commons, told BBC Radio on Monday.
But expelling Russian intelligence agents would mean that Britain would lose some of its own agents in Moscow, which would have steep costs for London, according to John Bayliss, who retired in 2010 from the Government Communications Headquarters, Britain’s electronic intelligence agency.
“It will cut off a flow of intelligence you have had for years,” he said. “That will stop you gaining intelligence in future years, which would be critical.”
Mr. Skripal and his daughter remained in critical condition on Monday, more than a week after being poisoned in Salisbury, where Mr. Skripal had lived quietly for years. The pair were found incoherent on a park bench, and a police officer who made contact with the nerve agent when he tried to help the Skripals, Detective Sgt. Nick Bailey, was also hospitalized in serious condition.
While working for Russian military intelligence, Mr. Skripal became a double agent, selling secrets to Britain. He was found out, convicted and sent to a Russian prison in 2006. In 2010, he was freed and sent to Britain in a spy swap with the West.