It added, “The British side is warned that, in the case of further actions of an unfriendly character toward Russia, the Russian side reserves the right to take other answering measures.”
The Kremlin delayed its response for three days until a day before national elections on Sunday, for which Mr. Putin has campaigned while casting himself as a defender of Russia against Western aggression.
The spy, Sergei V. Skripal, and his daughter, Yulia Skripal, were found unresponsive on a park bench in the cathedral city of Salisbury, England, after being attacked on March 4. British officials said the lethal nerve agent, Novichok, had been created in the Soviet Union in the 1970s and ‘80s.
The Kremlin has flatly denied any involvement in the attack, even as state television announcers have pointedly referred to the poisoning as a warning to traitors.
The case has roiled relations between the two countries, with Britain announcing that in addition to other measures, no ministers or members of the royal family would attend the World Cup hosted by Russia this summer.
Mr. Bristow, the British ambassador, told journalists in Moscow on Saturday, “We will always do what is necessary to defend ourselves, our allies and our values against an attack of this sort.”
The diplomatic crisis, he added, “has arisen as a result of an appalling attack in the United Kingdom, the attempted murder of two people using a chemical weapon developed in Russia and not declared by Russia” with the Organization for the Prohibition of Chemical Weapons, as is required by treaty.
The tit-for-tat expulsions were the second such episode following geopolitically related poisonings in Britain.
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After the British government blamed a Russian agent for adding a lethal dose of the radioactive element polonium-210 to tea sipped by Alexander V. Litvinenko, a former Russian Security Service officer, Britain expelled four Russian diplomats in 2007, and Russia responded in kind.
At a going-away party in Moscow for the expelled Britons, the “James Bond” theme music played on a loop.
In December 2016, the Obama administration expelled 35 Russian diplomats in retaliation for Moscow’s meddling in the United States presidential election that year. Russia, in turn, ordered the United States to cut its diplomatic presence in Russia to the same level as Russia’s in America, a reduction of about 700 positions, though these included support staff.
This time, Britain’s swift response seems to have played into the hands of President Vladimir V. Putin in domestic Russian politics. On Saturday, the British Foreign Office said in statement, “In light of Russia’s previous behavior, we anticipated a response of this kind and the National Security Council will meet early next week to consider next steps.”
As the Russian elections have approached and the row with Britain escalated, Mr. Putin has cast himself as the protector of a country besieged by outside forces. In interviews released this past week, for example, he described previously undisclosed decisions he had made in tense moments during his long tenure as commander in chief.
In 2014, he said, he ordered the downing of a passenger plane suspected of having been hijacked by terrorists intent on crashing it into the opening ceremony of the Winter Olympics in Sochi, though it was a false alarm.
Mr. Putin said terrorists who seized a theater in Moscow in 2002 had intended to execute hostages on Red Square, but were thwarted when he ordered the release of a powerful sleeping gas on the premises — though this killed more than 100 hostages.
The Russian news media is generally portraying the poisoning of the Skripals in Britain as a plot against Russia, one intended, improbably, to derail Mr. Putin’s election chances — he is widely expected to win easily — or as revenge for Britain’s having lost a bid years ago to host the World Cup soccer tournament.
“A Nervous Paralytic Reaction,” the state newspaper, Rossiyskaya Gazeta, proclaimed about the British response.” “Theresa May Has Poisoned Relations Between London and Moscow,” a headline in Izvestia declared.
Komsomolskaya Pravda, another newspaper, quoted a chemist, Zhores Medvedev, as saying that the attempted murders may have been the work of exiled Russian oligarchs who had somehow obtained the rare poison.
“This is a provocation,” he said. “The campaign that Britain has fomented with the World Cup soccer tournament, which instead of England will take place this year in Russia, always leads to certain suspicions.”
After the expulsions, Mrs. May said that “Russia’s response doesn’t change the facts of the matter: the attempted assassination of two people on British soil, for which there is no alternative conclusion other than that the Russian state was culpable.”