British Prime Minister Rishi Sunak demanded that the Iranian government provide answers about the death and burial of British-Iranian dual national Alireza Akbari, executed by Tehran earlier this month.
"The regime is prolonging the suffering of the family, and it is sadly typical of that disregard for basic human dignity," Sunak told parliament.
"Iran must now provide answers about the circumstances of his death and burial."
On January 14, Tehran defied pleas from the UK and the US and executed Akbari, prompting London to impose a new round of sanctions on Iranian officials earlier this week.
Akbari served as deputy defense minister when Ali Shamkhani was minister from 1997 to 2005. He had been a close ally of Shamkhani, the secretary of Iran's Supreme National Security Council.
Labour lawmaker Andy Slaughter said he was due to meet a British foreign office minister on Thursday along with the family of Akbari, who had lived in Slaughter's constituency, to seek the government's help at "this time of greatest suffering."
The UK summoned its ambassador to Tehran, Simon Shercliff, who announced his return Wednesday to Iran to continue his duties.
Shercliff tweeted about graffiti that covered the wall of the British embassy in Tehran and was drawn by supporters of the regime.
After imposing the sanctions, Foreign Secretary James Cleverly said in a statement: "Those sanctioned today, from the judicial figures using the death penalty for political ends to the thugs beating protesters on the streets, are at the heart of the regime's brutal repression of the Iranian people."
Cleverly faced pressure from members of the House of Commons, demanding the inclusion of the Iranian Revolutionary Guard Corps (IRGC) on the list of terrorist organizations after London and its allies in the EU announced they were considering the measure.
Earlier, the Sunday Telegraph stated that the UK was reconsidering its support for the 2015 Iran nuclear deal after the execution of Akbari.
The newspaper said, citing senior government people it didn't identify, that circumstances have changed since the negotiation process for the revival of the accord began, and Britain is reviewing its options regarding involvement.
The Iranian authorities denied that Akbari's execution resulted from a conflict of factions within Iran.
They also refused to confirm speculation that Shamkhani's duties would be terminated amid reports that Supreme Leader Ali Khamenei rejected President Ibrahim Raisi's team's attempts to dismiss Shamkhani.