Authorities in Russia are investigating allegations that a former Russian tax inspector spied for Britain.
The Federal Security Service (FSB) said Vyachaslav Zharko was recruited by agents from Britain's MI6 foreign intelligence service.
Correspondents say the move is part of a row between Britain and Russia over the murder in London last year of Russian ex-agent Alexander Litvinenko.
On Thursday Russia officially refused to extradite a suspect in the killing.
British prosecutors have alleged that Russian businessman Andrei Lugovoi gave Mr Litvinenko a fatal dose of radioactive polonium-210.
Mr Lugovoi denies the charges.
The FSB says Mr Zharko was recruited by MI6 through the mediation of Mr Litvinenko and an exiled Russian business tycoon, Boris Berezovsky.
Mr Berezovsky, an outspoken critic of Russian President Vladimir Putin, lives in London where he has refugee status.
For years, Russia has been pressing Britain without success for the multi-millionaire's extradition to stand trial on corruption charges.
The FSB said Mr Zharko admitted spying for Britain after turning himself in because he feared for his life, AFP news agency reported.
The British Foreign Office has refused to comment on the allegations.
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