Rishi Sunak has condemned Iran’s “callous and cowardly” decision to execute British-Iranian dual national Alireza Akbari.
The Prime Minister said he was “appalled” by Tehran’s “barbaric regime” after its state media announced the killing.
Foreign secretary James Cleverly vowed that the action taken by Iran would “not stand unchallenged” by the UK.
Iranian state media on Saturday announced that Mr Akbari had been put to death.
Mr Akbari was an Iranian former deputy defence minister who was arrested in 2019 and accused of espionage for MI6 related to past nuclear talks between Iran and western nations, according to reports.
He denied the charge and said he was tortured and forced to confess on camera to crimes he did not commit, BBC Persian reported.
Sunak, writing on Twitter, said: “I am appalled by the execution of British-Iranian citizen Alireza Akbari in Iran.
“This was a callous and cowardly act, carried out by a barbaric regime with no respect for the human rights of their own people.
“My thoughts are with Alireza’s friends and family.”
Also taking to Twitter, Cleverly said: “Iran has executed a British national.
“This barbaric act deserves condemnation in the strongest possible terms.
“This will not stand unchallenged. My thoughts are with Alireza Akbari’s family.”
The foreign secretary had warned Tehran on Friday that London was watching the case “closely” and called on the Middle East nation not to follow through with its threat.
But a statement from the Iranian Students’ News Agency less than a day later said the execution had been carried out.
“The sentence of Alireza Akbari, the son of Ali with dual Iranian-British citizenship, who was sentenced to death on charges of corruption in the land and widespread action against the country’s internal and external security through espionage for the intelligence apparatus of the British government, was executed,” the news agency said.
Earlier this week, Mr Akbari’s wife Maryam told BBC Persian she was invited to a “final meeting” at the prison where he was put in solitary confinement.
Commons Foreign Affairs Committee chairwoman Alicia Kearns previously accused the Iranians of seeking to “weaponise” dual nationals as anti-government protests convulse the country.
The Conservative MP said Mr Akbari may have been singled out by the regime because of his closeness to a leading Iranian moderate who has led calls for dialogue and discussion.
Tehran has detained a number of dual and foreign nationals in recent years, including British-Iranian national Nazanin Zaghari-Ratcliffe, who was held in 2016 and released last year.