German federal prosecutors say the British man passed documents to Russian intelligence for cash.
A British man has been arrested in Germany on suspicion of spying for Russia.
German federal prosecutors said the man – named only as David S – worked at the British embassy in Berlin.
He allegedly passed documents to Russian intelligence “at least once” in exchange for an “unknown amount” of money.
He was arrested in Potsdam outside Berlin on Tuesday and his home and workplace have been searched.
A spokesman for Germany’s foreign ministry quoted by AFP news agency said Berlin was taking the case “very seriously”, and said spying by “a close alliance partner on German soil is unacceptable”.
The arrest was the result of a joint UK-German investigation, the statement read.
London’s Metropolitan Police have confirmed the arrest of a 57-year-old British national in Germany, as well as the involvement of the Met’s Counter Terrorism Command.
German authorities are in charge of the investigation, but officers will continue to work with German counterparts, the police said.
The man is due to appear before an investigating judge on Wednesday.
Prosecutors said he was hired as a local staff member at the embassy.
German and UK relations with Russia are already strained after several high-profile incidents in recent years.
The UK accused Russia of being responsible for a 2018 nerve agent attack in Salisbury against Sergei and Yulia Skripal – an accusation Russia has repeatedly denied.
Both survived the attack. But Dawn Sturgess, 44, died after she came into contact with a perfume bottle thought to have been used in the incident and then discarded.
Germany meanwhile treated Russian dissident Alexei Navalny after he was poisoned with the Novichok nerve agent on a Russian flight last year. Russian authorities have also repeatedly denied their involvement in this incident.
Last month German Chancellor Angela Merkel and US President Joe Biden vowed to stand together “against Russian aggression”.