Iran will execute Swedish-Iranian citizen Ahmadreza Djalali by May 21 at the latest, the semi-official ISNA news agency reported on Wednesday.
Mr Djalali, 50, who used to live in Stockholm where he worked at the Karolinska medical institute, was arrested during a visit to Iran in April 2016.
He was sentenced to death 18 months later after being found guilty of passing information about two Iranian nuclear scientists to Israel's Mossad intelligence agency that led to their assassinations.
An expert in disaster medicine, Mr Djalali has said he is being punished for refusing to spy for Iran while working in Europe.
He was granted Swedish citizenship in February 2018 while in prison, in an unsuccessful bid to persuade Iran to commute his sentence.
In 2017, Iran's state TV aired a report in which Mr Djalali appeared to confess relaying information to a foreign intelligence service about Iranian nuclear scientists, who were later assassinated in 2010.
He did not name the country, but the broadcast carried images of a Swedish ID card and Rome's Colosseum, AFP reported.
Mr Djalali said in the footage that before he left Iran he had worked on a project for Iran's Defence Ministry, which may have been why his recruiters — allegedly officers from Israel's Mossad spy agency posing as European authorities — sought him out.
On Monday, Iran's Ministry of Foreign Affairs said it was summoning Sweden’s ambassador to condemn the trial of former Iranian official Hamid Noury.
The 61-year-old has been on trial in Stockholm since last August.
Arrested in 2019, he stands accused of being involved in a wave of mass killings in 1988, at the end of the bloody Iran-Iraq war.
Rights groups say as many as 30,000 people were killed, although estimates of the final death toll vary significantly.
Numerous Iranians who hold foreign passports and have resided outside Iran for most of their lives, have been detained on spy charges over the past few years, while visiting the Middle Eastern country.