“This morning, a Swiss citizen committed suicide in Semnan prison,” a statement on Mizan’s website, attributed to Mohammad Sadegh Akbari, Semnan’s chief justice, said.

The statement added that he had been “arrested by Iranian security forces for espionage” and the case was “under investigation”.

In recent years, Iran’s elite Revolutionary Guards have arrested dozens of dual nationals and foreigners, mostly on charges related to espionage and security.

Switzerland plays an important intermediary role between Washington and Tehran as it represents American interests in Iran and shares messages between the two countries.