Iran has executed British-Iranian national Alireza Akbari, after sentencing the former Iranian deputy defense minister to death on charges of spying for Britain.

Britain called the execution a barbaric act immediately after announcement by Iran on Saturday and said it would not go unpunished.

"I am appalled by the execution of British-Iranian citizen Alireza Akbari in Iran," British Prime Minister Rishi 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 also tweeted to say: "This barbaric act deserves condemnation in the strongest possible terms. This will not stand unchallenged."

British Foreign Secretary James Cleverly had said late on Friday Iran must not follow through with the execution - a call echoed by the US State Department. Britain had described the death sentence as politically motivated and called for his release.

Before the execution was announced, the US State State Department had joined the UK to condemn Akbari's death sentence.

"The United States echoes the British government's strong call for Iran not to proceed with this execution, and to release Mr. Akbari immediately. The charges against Ali Reza Akbari, and his sentencing to execution, were politically motivated. His execution would be unconscionable," Vedant Patel State Department Principal Deputy Spokesperson said.

Mizan website of Iran’s hardliner Judiciary said in a Tweet early on Saturday the sentence had been carried out, without saying when.

"Alireza Akbari, who was sentenced to death on charges of corruption on earth and extensive action against the country’s internal and external security through espionage for the British government's intelligence service ... was executed,” it said.

The report accused Akbari, arrested in 2019, of receiving 1,805,000 euros, 265,000 pounds, and $50,000 for spying.

In a 30-minute audio recording from Akbari received by Iran International on Thursday, he is heard saying that after thousands of hours of interrogation he was forced to confess to acts he never committed.

"After more than 3,500 hours of torture, psychedelic drugs, and physiological and psychological pressure, they took away my will. They drove me to the brink of madness. They instilled in me what they wanted and forced me to make false confessions at gunpoint and threats of death," he said.

Iranian state media broadcast a video on Thursday that they said showed that Akbari played a role in the 2020 assassination of Iran's top nuclear scientist, Mohsen Fakhrizadeh, killed in a 2020 attack outside Tehran which authorities blamed at the time on Israel.

In the video, Akbari did not confess to involvement in the assassination but said a British agent had asked for information about Fakhrizadeh.

Iran’s state media often airs forced confessions by suspects in politically charged cases.

London-Tehran ties have deteriorated in recent months as efforts have stalled to revive Iran's 2015 nuclear pact, to which Britain is a party.

Britain has also been critical of the Islamic Republic's violent crackdown on antigovernment protests, sparked by the death in custody of a young Iranian-Kurdish woman in September.

A British foreign office minister said on Thursday that Britain was actively considering proscribing Iran's Revolutionary Guard as a terrorist organization but has not reached a final decision.

Iran has issued dozens of death sentences as part of the crackdown on the unrest, executing at least four people.

Akbari was a close ally of Ali Shamkhani, now the secretary of Iran’s Supreme National Security Council, who was defense minister from 1997 to 2005, when Akbari was his deputy.

A source close to the Islamic Revolutionary Guard Corps has provided documents to Iran International showing that accusations against Akbari and his death sentence were aimed at weakening Shamkhani’s position in the clerical regime. It seems that President Ebrahim Raisi, Intelligence Minister Esmail Khatib and Interior Minister Ahmed Vahidi are exerting pressure to remove Shamkhani from the post.

With reporting by Reuters