The execution of British-Iranian man Alireza Akbari, who had been sentenced to death, has been widely condemned.
The ex-deputy Iranian defence minister was arrested in 2019 and convicted of spying for the UK, which he denied.
UK Prime Minister Rishi Sunak said his execution was a “callous and cowardly act, carried out by a barbaric regime”.
France summoned Iran’s top diplomat in Paris, warning that Tehran’s repeated violations of international law could not go unanswered.
Meanwhile, the UK has imposed sanctions on Iran’s Prosecutor General, saying it would hold the regime to account “for its appalling human rights violations”.
“Sanctioning him today underlines our disgust at Alireza Akbari’s execution,” UK Foreign Secretary James Cleverly said.
Iran summoned the British ambassador in Tehran over what it called Britain’s “unconventional interventions”.
The Iranian judiciary’s official news outlet Mizan reported on Saturday that Mr Akbari, 61, had been hanged. It did not specify the date when the execution took place.
Iran posted a video of Mr Akbari earlier this week showing what appeared to be forced confessions, and after the country’s intelligence ministry had described the British-Iranian as “one of the most important agents of the British intelligence service in Iran”.
However, BBC Persian broadcast an audio message on Wednesday from Mr Akbari in which he said he had been tortured and forced to confess on camera to crimes he did not commit.
Mr Akbari’s family had been asked to go to his prison for a “final visit” on Wednesday and his wife said he had been moved to solitary confinement.
Human rights group Amnesty International called on the UK to investigate claims Mr Akbari was tortured before his death. The group accused Iran of showing “pitifully little respect” for human life.
Dr Sanam Vakil, Iran expert at international affairs think tank Chatham House, said Mr Akbari’s death would be used by the Iranian regime to suggest a “heavy outside hand” was stoking the anti-government unrest – linking the protests with the accusation that Western nations were trying to “destabilise the Islamic republic”.
“Keeping the narrative of the West being involved is a way to maintain unity among the political establishment,” she told BBC Radio 4’s Today programme.
Ties between the UK and Iran have deteriorated in recent months since London imposed sanctions on Iran’s morality police and other top security figures, in response to the country’s violent crackdown on anti-government protesters.
Iran has arrested dozens of Iranians with dual nationality or foreign permanent residency in recent years, mostly on spying and national security charges.
British-Iranian citizens Nazanin Zaghari-Ratcliffe and Anoosheh Ashoori were released and allowed to leave Iran last year after the UK settled a longstanding debt owed to Tehran.