Three men have been arrested by Metropolitan Police officers on suspicion of assisting Russia's foreign intelligence service as part of an ongoing national security investigation.
The suspects, who are aged 48, 45, and 44, were detained at various addresses in west and central London.
They were held under the National Security Act, as searches continued at properties where they were picked up and an additional location in the capital.
The trio have been bailed until January, Scotland Yard said in a statement.
Commander Dominic Murphy, head of Counter Terrorism Policing London: "Anyone who might be contacted by and tempted into carrying out criminal activity on behalf of a foreign state here in the UK should think again.
"This kind of activity will be investigated and anyone found to be involved can expect to be prosecuted and there are potentially very serious consequences for those who are convicted."
The arrests come as authorities clamp down on the increasing number of "proxies" being recruited by rouge states.
Earlier this year, six Bulgarian nationals were jailed for spying for the Kremlin in Britain and across continental Europe.
The group conducted surveillance for Vladimir Putin’s nation "on an industrial scale", targeting journalists, dissidents and Ukrainian troops.
And last month, two men and a woman were arrested in Essex on suspicion of assisting the Russian government.
Meanwhile, MI6 recently launched a dedicated portal on the dark web in the hope of attracting spies online.
MI5 boss Sir Ken McCallum said the intelligence agency had seen a 35 per cent increase in the number of individuals it was probing in the last year.
