Russia's Federal Security Service, or FSB, said the two diplomats had provided false personal data while seeking permission to enter the country and had engaged in alleged intelligence and subversive activities that threatened Russia's security.
It did not offer any evidence.
In an FSB statement quoted by the state news agency RIA Novosti, a decision had been made to revoke the diplomats' accreditations and they had been ordered to leave Russia within two weeks.
The Russian foreign ministry said in a separate statement that it summoned a British embassy official.
"Moscow will not tolerate the activities of undeclared British intelligence officers on Russian territory," it said.
The Foreign Office in London said: "This is not the first time that Russia has made malicious and baseless accusations against our staff."
It did not say whether the UK planned any retaliatory moves.
The FSB in 2024 accused seven British diplomats of spying.
Six expulsions were announced in September and one more in November.
The UK called the moves at the time "baseless".
The expulsions came amid soaring tensions over the war in Ukraine and after London decided to revoke the credentials of an attache at the Russian embassy and limit Moscow's diplomatic activities in Britain.
In February, London expelled a Russian diplomat in response to the November expulsion.
In May 2024, the UK expelled Russia's defence attache in London, alleging he was an undeclared intelligence officer, and it closed several Russian diplomatic properties in Britain that it said were being used for spying.
Days later, Russia reciprocated and expelled Britain's defence attache.
Expulsions of diplomats - both Western envoys working in Russia and Russians in the West - have become increasingly common since Moscow launched its invasion of Ukraine in 2022.
In 2023, the Russian news outlet RBC said Western countries and Japan had expelled a total of 670 Russian diplomats between the start of 2022 and October 2023, while Moscow responded by expelling 346 diplomats.
According to RBC, that was more than in the previous 20 years combined.
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