Britain’s diplomatic feud with Russia escalated on Wednesday after the British government announced it would expel a senior Russian diplomat who officials claim is an “undeclared” military intelligence officer, and also shut down several Russian diplomatic facilities in the country.
The government accused Russia’s foreign intelligence service, the Federal Security Service, or F.S.B., of a pattern of “malign activity” in Britain and Europe, including hacking and leaking trade documents relating to the United States, and targeting of British lawmakers through malicious email campaigns.
James Cleverly, the British home secretary, told Parliament that the government was announcing the retaliatory measures “to make clear to Russia that we will not tolerate such apparent escalations.”
Britain’s action came two days after the Russian Foreign Ministry said it had summoned the British ambassador to Moscow to lodge a “strong protest” over remarks the British foreign secretary, David Cameron, had made about Ukraine’s using weapons supplied by Britain to strike Russian territory.
The British Foreign Office disputed that the envoy, Nigel Casey, had been summoned, characterizing it instead as a “diplomatic meeting” in which, it said, he had “reiterated the United Kingdom’s support for Ukraine in the face of unprovoked Russian aggression.”
Whatever the diplomatic nuances, it is clear Britain’s relations with Russia — already among the most brittle of any between Moscow and a NATO member — are tumbling into an even deeper freeze. Russia’s Foreign Ministry vowed a “tough and measured” response, according to a statement.
“We consider extremely irresponsible and categorically unacceptable the claims of the British about the alleged involvement of our country in certain malicious actions,” said Maria V. Zakharova, a spokeswoman for the ministry.
Mr. Cleverly said Britain would expel the diplomat, a defense attaché whom he did not identify, and put new restrictions on visas for Russian diplomats, limiting the amount of time they can spend in Britain. He said Russia had tried to destabilize a British research group that focused on combating disinformation.
Britain will also strip the diplomatic status of Russian-owned properties, including Seacox Heath, a house in East Sussex that the Russian embassy has used as a weekend retreat for its staff, and a trade and defense office in Highgate, in London. Mr. Cleverly said these facilities “have been used for intelligence purposes.”
In addition to its operations in Britain, Mr. Cleverly accused Russia of plotting to sabotage German military aid for Ukraine and of carrying out espionage in Italy and Bulgaria. Among its activities, he said, were cyber and disinformation campaigns and the jamming of signals to disrupt civil aviation.
In a statement, Mr. Cameron, the foreign secretary, said, “Since the illegal invasion of Ukraine, Russia’s attempts to undermine U.K. and European security have become increasingly brazen.”
“These measures are an unequivocal message to the Russian state — their actions will not go unanswered,” he said.
It was the second time this week that Britain accused a hostile foreign actor of cyberattacks. In the first announcement on Tuesday — involving a large-scale breach of the payroll data of British military personnel — it did not name the country behind the attack. Lawmakers pointed the finger at China.
Asked whether Chinese hackers were responsible, Prime Minister Rishi Sunak of Britain said China was “acting in a way that is more authoritarian at home, assertive abroad.” Britain, he added, was confronting “an axis of authoritarian states, including Russia, Iran, North Korea and China.”
Britain’s tensions with Russia date back decades. But they deepened in 2018, after a former Russian intelligence agent and his daughter were poisoned with a nerve agent in Salisbury, England. Britain blamed Russia’s military intelligence and expelled 23 diplomats it branded as undeclared intelligence officers.
In 2020, a British parliamentary committee concluded that Russia had mounted a prolonged campaign to undermine Britain’s democracy — using tactics from disinformation, meddling in elections, funneling dirty money and employing members of the House of Lords. Russia dismissed the conclusions as “Russophobia.”
Britain was one of the first NATO countries to supply weapons to Ukraine, as it faced a threat from Russian troops in early 2022. A succession of British leaders, including Mr. Sunak and one of his predecessors, Boris Johnson — have staked out positions as stalwart defenders of Ukraine against Russia.
Mr. Cameron, who also served as prime minister, lobbied Republicans in Washington to extend American military aid to Ukraine, even paying a visit to former President Donald J. Trump at Mar-a-Lago, his estate in Palm Beach, Fla.
Mr. Cameron provoked a furious reaction in Moscow when he suggested during a recent visit to Kyiv, the Ukrainian capital, that Ukraine “absolutely has the right to strike back at Russia,” including with British weapons. The United States and other suppliers of arms have generally discouraged strikes on Russian territory because of fears that it could draw the West further into the war.
In its statement after calling in the British ambassador, the Russian Foreign Ministry said Mr. Cameron was “de facto recognizing his country as a party to the conflict.”
“The ambassador was called upon to think about the inevitable catastrophic consequences of such hostile steps from London,” the ministry said.
Britain’s suspicion of Russia extends even to the British royal family. Researchers in Britain reported that a Kremlin-linked disinformation network had spread spurious reports on social media about the health problems of Catherine, Princess of Wales, with a goal of inflaming divisions and eroding trust in institutions.
Catherine disclosed in March that she had been diagnosed with cancer, which quieted those rumors. But with Britain only months away from an anticipated general election, researchers and government officials are on guard for evidence of further Russian efforts to destabilize the country.
Anton Troianovski contributed reporting.
