British prosecutors said on Thursday they had authorised charges to be brought against five Bulgarian nationals accused of spying for Russia for almost three years.
The three men and two women are accused of "conspiring to collect information intended to be directly or indirectly useful to an enemy for a purpose prejudicial to the safety and interest of the state" between Aug 30, 2020 and Feb 8, 2023, the Crown Prosecution Service (CPS) said.
The alleged spies were named as Orlin Roussev, 45, Bizer Dzhambazov, 41, Katrin Ivanova, 31, Ivan Stoyanov, 31, and Vanya Gaberova, 29, all Bulgarian nationals who lived in London and Norfolk.
They are due to appear at Westminster Magistrates' Court on Sept 26. There was no immediate response from the Russian embassy in London.
"The CPS has authorised a charge of conspiracy to conduct espionage against three men and two women suspected of spying for Russia," said Nick Price, head of the CPS Special Crime and Counter Terrorism Division.
"Criminal proceedings against the five individuals are active and they each have the right to a fair trial."
The action followed an investigation by the London Metropolitan Police's Counter Terrorism Command, the CPS said.
Roussev, Dzhambazov, and Ivanova had already been charged in February with identity document offences, the CPS said.
Britain has been seeking to take tougher action on external security threats and potential spies, and in July passed a national security law, aiming at overhauling its means of deterring espionage and foreign interference with new tools and criminal provisions.
At the time, the government labelled Russia as "the most acute threat" to its security.
Last November, Britain's domestic spy chief said more than 400 suspected Russian spies had been expelled from Europe, striking the "most significant strategic blow" against Moscow in recent history.
British police have previously charged three Russians, who they say are GRU military intelligence officers, with the 2018 attempt to murder former double agent Sergei Skripal with the military-grade nerve agent Novichok. Two were charged in 2018 and the third in 2021.