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.

It was intelligence-led and had been going on for some time leading up to the arrest, the BBC's Security Correspondent Gordon Corera says. MI5 and other UK agencies, as well as British police, had been working with the Germans to learn as much as they could about the alleged activity.

The head of MI5 is to urge the public to be as vigilant about threats from "hostile states" as from terrorism. These include disruptive cyber-attacks, misinformation, espionage and interference in politics - and are usually linked to Russia and China.

In a speech on Wednesday, Ken McCallum will say these "less visible threats... have the potential to affect us all". He will say these threats are affecting UK jobs and public services and could even lead to a loss of life.

The head of the Security Service wants to challenge the idea that activity by so-called "hostile states", usually taken to mean primarily Russia and China, only affects governments or certain institutions. Instead, he will argue, in an annual threat update, that the British public are not immune to the "tentacles" of covert action by other states.