Relations were already rocky over Moscow’s roles in the wars in Syria and Ukraine, its annexation of Crimea, its meddling in elections in the United States and elsewhere, the assassination of Kremlin foes in Russia and abroad, cyberattacks and disinformation campaigns against other countries and what Western officials have described as a broad, largely covert effort to destabilize and discredit liberal democracies.
Russia as a whole and many powerful Russians individually are already under economic sanctions by the West, and London has vowed to tighten its scrutiny and control of the vast Russian wealth — much of it held by allies of President Vladimir V. Putin — that has flowed into Britain in recent years. Britain has also said it will re-examine several suspicious deaths of Kremlin opponents.
Mr. Putin and his government have denied any involvement in the March 4 attack on Sergei V. Skripal and his daughter, Yulia, and have tried to cast blame on Britain, the United States, Ukraine, the Czech Republic and others.
The Skripals were found unconscious in a busy shopping area in the small English city of Salisbury, where Mr. Skripal lives. He remains hospitalized in critical condition, but his daughter is showing improvement, British officials announced on Thursday. British officials say that hundreds of people could have been exposed to Novichok, the toxin used against the Skripals.
Mr. Lavrov said that Russia had called for a meeting next Tuesday of the Organization for the Prohibition of Chemical Weapons to “establish the truth” with respect to the Skripal case.
Prime Minister Theresa May of Britain and her government contend that the pair were poisoned with one of an extremely powerful class of nerve agents known as “Novichok,” developed by Soviet scientists in the 1970s and ’80s. They claim to have solid evidence that Russia was probably behind the attack, and that Mr. Putin himself probably approved it.
The British government has not made its evidence public, but has shared it with major allies, who have said that they agree with London’s conclusions.
President Trump, who has long been loath to criticize Mr. Putin or his government, has made no public statement on the nerve-agent attack or who was to blame for it. But officials in his administration have publicly backed Mrs. May’s statements, and on Monday the president ordered the expulsion of 60 Russian officials who work in the United States, and the closing of the Russian consulate in Seattle.
More than 20 other countries, primarily European, also announced expulsions on Monday, and a few more joined in on Tuesday, as did NATO headquarters in Brussels. The expulsions were a remarkable show of international unity and coordination, in solidarity with Britain, which had already forced 23 Russian officials to leave the country; Moscow responded by expelling 23 Britons.
In all, 27 countries are ejecting more than 150 Russians, including people listed by their embassies and consulates as diplomats, and military and cultural attachés. Western officials say that many of the Russians are spies and that the expulsions will hinder Russian espionage efforts.
Mr. Skripal, a former colonel in Russian military intelligence who was imprisoned in Russia for selling secrets to the British, was sent to Britain in 2010 as part of a spy swap. Why he would be targeted years later is unclear, but political and security analysts have said that the attack served as a warning to those who would cross Mr. Putin that, even in exile, they are never beyond the Kremlin’s reach.
On March 12, Nikolai A. Glushkov, a former Russian business executive and critic of the government, died suddenly at his home in London, and the police are treating the case as a murder investigation.