An employee of the British embassy in Berlin is under arrest for allegedly providing documents to Russian intelligence in exchange for money, German and British law enforcement said on Wednesday.
An investigating judge at the Federal Court of Justice in Karlsruhe was to decide later in the day on whether the Briton should be remanded in custody, German federal prosecutors said in a statement.
In London, the Home Office confirmed the arrest, to which joint investigations by German and British authorities contributed.
The Foreign Office in Berlin stressed that it was taking the case very seriously. "Intelligence spying on a close alliance partner on German soil is not something we can accept," a spokesperson said. "That is why we will follow the further investigations by the federal prosecutor general very closely."
According to the authorities, the man is under suspicion of intelligence agent activities. He is said to have worked for the Russian secret service since November, at the latest. On at least one occasion, he passed on material to the service that he had obtained in the course of his work.
The man, who is only being identified as David S under Germany's strict privacy laws, was arrested on Tuesday in Potsdam, outside of Berlin, on suspicion of having worked for a foreign intelligence agency since November, the German side said.
It was not clear how much money he received in return from the representative of Russian intelligence service with whom he met, read the statement, which described the suspect as a local hire at the British embassy in the German capital.
The accused's home and workplace were also searched.
The arrest was the result of joint investigations by German and British authorities. The British gave the suspect's age as 57.
"The man was arrested in the Berlin area on suspicion of committing offences relating to being engaged in 'intelligence agent activity' (under German law)," the Metropolitan Police in London said in a statement.
"Primacy for the investigation remains with German authorities. Officers from the Counter Terrorism Command continue to liaise with German counterparts as the investigation continues," the statement said.
The case is explosive for the government in London. Bilateral relations with Russia are bad. Only a few weeks ago, there was an incident in which Russian border guards tried to divert a British warship away from the annexed Crimean Peninsula in the Black Sea by firing warning shots and dropping bombs.
In addition, Britain accuses Russia of ordering the poison attack on double agent Sergei Skripal and his daughter in Salisbury, England, in 2018. Moscow is also blamed for the poisoning of Russian ex-agent Alexander Litvinenko in London in 2006.
"An individual who was contracted to work for the government was arrested yesterday by the German authorities," Britain's Press Association (PA) quoted a British government spokesperson as saying. "It would not be appropriate to comment further as there is an ongoing police investigation."
Local staff are employees who are not sent by the government from their home country, but are hired at the place of deployment.