The UK government has always rejected the link drawn by authorities in Tehran between the money - owed for an order of tanks from Britain which were never delivered - and the continued detention of Ms Zaghari-Ratcliffe and other UK-Iranian dual nationals.
But the detained mother-of-one’s husband Richard Ratcliffe, who recently completed a 21-day hunger strike to raise awareness of Nazanin’s plight, has demanded answers from ministers on the cash.
And former foreign secretary Jeremy Hunt today demanded to know whether it was being withheld because of fears that making the payment would breach international sanctions or because of pressure from the US.
The money was paid to the UK by the former Shah of Iran before he was deposed in the country’s 1979 revolution, but Britain refused to hand over the tanks to the new Islamic Republic.
In an appearance before the House of Commons Liaison Committee, Mr Johnson was challenged by Mr Hunt over what obstacles prevent the debt being paid.
He asked whether the UK could follow the example of former US president Barack Obama, who authorised a flight to Iran carrying wooden pallets bearing $400m of cash in various currencies on the same day that Tehran released four American prisoners and formally implemented its nuclear deal.
“If you can’t use a bank to repay it, for various reasons, why can’t we do what President Obama did in January 2016 and fly a crate of cash to Tehran and just repay that debt?” Mr Hunt asked the prime minister.
Mr Johnson replied: “It is certainly worth considering. But as you know there are complexities attached.”
The PM said the government was working “as hard as we can” to secure Nazanin’s release.
Giving evidence to the committee, Mr Johnson said it “breaks my heart” that he could not give an assurance she would be home in time for Christmas.
“I continue to be horrified by the ordeal she has been through. I have nothing but admiration for the way in which she has dealt with it. It has been unbearable to witness and I know how much she wants to come home and to see her family,” he said.
“I can tell you we are working as hard as we can to ensure that we deliver that. It is not easy. If I could tell Nazanin now that we’d have her home for Christmas, I certainly would.
“It breaks my heart that I can’t make that promise. We will continue to do what we can.”
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