The foreign secretary has condemned the planned execution of a British-Iranian dual national by Iran, calling it "a politically motivated act by a barbaric regime".
James Cleverly has appealed for the release of Alireza Akbari, a former senior defence official in Iran accused of working for British intelligence.
The Iranian judiciary claimed Mr Akbari, who was deputy defence minister under former president Mohammad Khatami until 2001, was a "key spy" for the British government, the semi-official Tasnim news agency reported.
It said Iranian intelligence unmasked the alleged spy by feeding him false information and described him as "one of the most important infiltrators of the country's sensitive and strategic centres".
Mr Cleverly tweeted: "Iran must halt the execution of British-Iranian national Alireza Akbari and immediately release him.
"This is a politically motivated act by a barbaric regime that has total disregard for human life."
Mr Akbari claims he was tortured and given mind-altering drugs and forced to confess to crimes he did not commit.
His wife Maryam has said she has been asked to attend a "final visit" at the prison where he is being held, suggesting his execution may be imminent, according to BBC Persia.
Commons Foreign Affairs Committee chair Alicia Kearns also condemned the planned execution, accusing the regime in Tehran of reacting because it felt "cornered" as a result of international sanctions and anti-government protests.
"It is another horrifying example of the Iranian regime, because they feel they are cornered, because there is such significant pressure from sanctions, weaponising British nationals and industrialising hostage-taking," she told the BBC Radio 4 PM programme.
Following the recent executions of anti-government protesters, Ms Kearns said Western countries should respond with fresh sanctions against the regime.
"Every time the Iranian government assassinates an individual involved in the protests, there should be sanctions applied by Western government the next day to make the point that we stand by those protesting for the basic rights of Iranians, otherwise we are showing no meaningful support to them," she said.
A Foreign Office spokesperson said: "We are supporting the family of Mr Akbari and have repeatedly raised his case with the Iranian authorities."
"Our priority is securing his immediate release and we have reiterated our request for urgent consular access."