Iran has executed Ali Reza Akbari, a British-Iranian dual national accused of spying for Britain, the country's judiciary announced on Saturday.
Mr Akbari, a former deputy defence minister of Iran, was convicted in Tehran of spying for MI6, the UK’s foreign intelligence agency, and accused of involvement in the assassination of Iran's senior nuclear scientist Mohsen Fakhrizadeh.
The Iranian judiciary's Mizan news agency said Mr Akbari was hanged “after going through all the legal procedures and confirming the court verdict in the Supreme Court”.
British Prime Minister Rishi Sunak condemned the execution as the act of a “barbaric regime”.
“I am appalled by the execution of British-Iranian citizen Ali Reza Akbari in Iran,” Mr Sunak said on Twitter.
“This was a callous and cowardly act, carried out by a barbaric regime with no respect for the human rights of their own people.”
British Foreign Secretary James Cleverly, who called on Friday for Iran to not carry out the death sentence, said Mr Akbari's execution “will not stand unchallenged”.
Mr Cleverly summoned Iran’s charge d’affaires in the UK and ordered the temporary withdrawal of British ambassador from Tehran as Britain also sanctioned the Iran’s prosecutor-general.
“Our response to Iran is not limited to today,” he warned.
In response, Iran summoned the British ambassador to protest against what it described as "unconventional interventions".
Britain and the United States have said Mr Akbari's case was politically motivated.
The US, France and Germany joined Britain in condemning Iran for carrying out Mr Akbari's death sentence. US State Department deputy spokesman Vedant Patel said on Friday that his execution would be "unconscionable”.
Iranian state media broadcast a video on Thursday that they said showed that Mr Akbari played a role in the 2020 assassination of Mr Fakhrizadeh in an attack outside Tehran. Iran had blamed that assassination on Israel.
In the video, Mr Akbari did not confess to involvement in the assassination but said a British agent had asked for information about Mr Fakhrizadeh.
The BBC's Persian service obtained an audio file in which Mr Akbari speaks about the torture he endured during 3,500 hours of interrogation to obtain his confession.
In the clip, he said: “I was given new clothes and asked to dye my hair to be released but then I was taken to a film studio and threatened with a gun to falsely confess.”
Iran’s state media often airs purported confessions by suspects in politically charged cases.
Ties between London and Tehran have deteriorated in recent months as efforts to revive Iran's 2015 nuclear pact, to which Britain is a party, have stalled.
Britain has also been critical of Iran's repression of anti-government protests, sparked by the death in September of a young woman arrested by its morality police. Iran has executed at least four protesters and dozens of others are facing charges that carry the death sentence.
France, another party to the nuclear deal along with the US, Germany, Russia, China and the European Union, said it condemned Mr Akbari's execution "in the strongest terms".
"The execution of Alireza Akbari is a despicable and barbaric act. His name adds to too long a list of victims of repression and the death penalty in Iran. Solidarity with the UK. Solidarity with the Iranian people," French President Emmanuel Macron said on Twitter.
The French Foreign Ministry said Iran's envoy in Paris was summoned to "express our indignation" and to warn that "repeated violations of international law by Iran will not remain unanswered particularly with regard to the treatment of foreign nationals held arbitrarily".
France has said at least seven of it citizens are being held by Iran, including some accused of spying.
Secretary of State Antony Blinken said the US stood firmly with the UK in condemning Iran’s execution of Mr Akbari.
"We mourn with his loved ones and will continue to hold Iran accountable for its sham trials and politicised executions," Mr Blinken wrote on Twitter.
German Foreign Minister Annalena Baerbock said Mr Akbari's execution was "yet another inhumane act by the Iranian regime".
"We stand with our British friends and will continue to closely co-ordinate our measures vis-a-vis the regime and our support for Iran's people," she said.
Mr Akbari had served as deputy defence minister under Iran's reformist president Mohammed Khatami and left the country after stepping down in 2001.
He settled in the UK, where he became a naturalised citizen and lived for more than a decade before his arrest in Iran in 2019.
His wife Maryam Samadi, who lives in Hammersmith, London, said the family decided against raising his case publicly in the hope he would be released on an internal appeal.
However, he was reportedly told three months ago his final appeal had been rejected.
She raised his case publicly this week after he was placed in solitary confinement and informed his execution was imminent.
She told The Guardian: “He is entirely innocent and the victim of political games inside the country.”