Boris Johnson on Wednesday declared that it is “worth considering” sending a crate of cash to Tehran to settle an historic ￡400 million debt.

       The Iranian regime has linked the payment of the outstanding sum owed by Britain to its ongoing detention of Nazanin Zaghari-Ratcliffe and other UK-Iranian dual nationals.

       The Prime Minister signalled his interest in novel ways to settle the arrears, but warned there were “complexities attached”.

       While the UK insists that the payment should not be treated as an issue linked to Iran’s treatment of mother-of-one Ms Zaghari-Ratcliffe, London does acknowledge the existence of the debt.

       It stems from a 1971 deal to sell Chieftain tanks to Iran. The UK took the cash up front, but refused to deliver the armoured fighting vehicles following the fall of the Shah.

       In September last year Ben Wallace, the Defence Secretary, said the Government “acknowledges there is a debt to be paid and continues to explore every legal avenue for the lawful discharge of that debt”.

       However, the UK has maintained that international sanctions levied on the Tehran authorities have so far made legal payment difficult.

       Appearing before the Commons liaison committee on Wednesday, Mr Johnson was asked by former foreign secretary Jeremy Hunt whether there was any obstacle to the UK simply repaying the debt.

       Mr Hunt suggested: “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 of a crate of cash to Tehran and just repay that debt?”.

       The Prime Minister replied: “It is certainly worth considering. But as you know there are complexities attached.”

       When the US delivered $400 million by plane to settle another old debt five years ago, the Iranian regime released four American prisoners.

       Another suggestion that has been advanced by Mr Hunt is for the UK to strike a deal with Iran to deliver ￡400 million worth of medicines, or another agreed commodity.

       Last Sunday Ms Zaghari-Ratcliffe’s husband Richard Ratcliffe ended a three-week hunger strike. He lost 12kg during his protest, which he staged in a tent camping outside the Foreign Office.