The actress Emma Thompson has joined the campaign calling for the release of Nazanin Zaghari-Ratcliffe, the British-Iranian mother who has been in jail in Iran for almost 19 months.
Speaking to ISNA on Thursday, Foreign Ministry Spokesman Bahram Qassemi denied British media reports that the United Kingdom government would pay the outstanding £450 million debt to Iran to help free Zaghari, who was arrested at Tehran's Imam Khomeini International Airport by the IRGC intelligence forces in April 2016.
"But Iran, particularly the Revolutionary Guard, will want something in return for her release, and payment of this debt could be it".
In the 1970s the Iranian Shah ordered and payed for 1,750 Cheiftan tanks from the UK.
Britain's debt to Iran dates from the 1970s, before the Islamic Revolution of 1979 toppled the US -backed Shah.
Sky's Defence Correspondence Alistair Bunkall said the money should not be considered a ransom payment.
The UK says it is unable to pay the debt because of worldwide sanctions on Iran.
Under a deal between Iran and six major powers in 2015, most sanctions on Iran were lifted a year ago, in exchange for Tehran curbing its nuclear programme. The amount was reportedly transferred to a bank account controlled by the High Court in 2002, but has not been given to Iran.
Dr Rowan said current Archbishop of Canterbury Most Rev Justin Welby, Cardinal Vincent Nichols or a senior Vatican figure could be well-placed to help secure Mrs Zaghari-Ratcliffe's freedom.
On Thursday, British Prime Minister Theresa May's spokesman denied there was any link between the debt and the charity worker's case. Speaking after meeting Boris Johnson, the Foreign Secretary, he said: "It is important that the United Kingdom honours its worldwide legal obligations, so that Iran can honour its legal obligations".