The suspect, named only as David S., was detained in Potsdam, near Berlin, after a joint investigation by German and British authorities.
The suspect is accused of forwarding documents obtained in the course of his work to a representative of a Russian intelligence service in return for an unknown amount of cash.
He is expected to appear before a judge at the Federal Court of Justice on Wednesday.
The 57-year-old suspect was arrested on Tuesday after an investigation involving Scotland Yard’s counter-terrorism officers.
In a statement, the Metropolitan Police said: “The man was arrested in the Berlin area on suspicion of committing offences relating to being engaged in ‘Intelligence Agent activity’ (under German law).
“Primacy for the investigation remains with German authorities. Officers from the Counter Terrorism Command continue to liaise with German counterparts as the investigation continues.”