Nazanin Zaghari-Ratcliffe is a “hostage” in Iran and was on the brink of being freed from jail in Iran almost two-and-a-half years ago until a planned deal collapsed, according to a new documentary.

The mother-of-one’s husband Richard Ratcliffe said the family had even been given a date of December 28 2017 for his wife’s return to the UK.

It is unclear why the deal fell through.

The revelations were made during a BBC Panorama investigation into the detention of British and other western citizens in Iran, which airs on Monday night on BBC One at 7.30pm.

It has long been claimed Mrs Zaghari-Ratcliffe is being held in order to force the UK into settling a multi-million pound dispute with Iran.

The debt dates back to the 1970s when the then-Shah of Iran paid the UK £400 million for 1,500 chieftain tanks.

It's part of Iran’s foreign policy to take people hostage who are innocent and then trade them later for some objective that they think advances their own objectives

When he was toppled in 1979, Britain refused to deliver the tanks to the new Islamic Republic but kept the cash, despite British courts accepting that the money should be repaid.

Panorama said that according to its sources, the Ministry of Defence has objected to the repayment.

Mrs Zaghari-Ratcliffe has said the Iranian authorities have told her that her release hinges on the money being returned.

But publicly the Iranian authorities have denied any link between the debt and Mrs Zaghari-Ratcliffe’s imprisonment.

The UK Foreign Office also said they do not “recognise any linkage” between the debt and her imprisonment, adding “it is unhelpful to suggest otherwise”.

Mrs Zaghari-Ratcliffe has so far served four years in prison and is currently on temporary release under effective house arrest at her parents’ home in Tehran due to the Covid-19 pandemic.

It is unclear whether she will be permanently freed or recalled to prison.

She was arrested at Tehran airport in April 2016 as she was returning to London with her daughter, following a visit to her parents to introduce them to their grandchild.

Mrs Zaghari-Ratcliffe was convicted of spying and jailed for five years.

She has been afforded diplomatic protection by the UK Government, which states that she is innocent and that her treatment by Iran failed to meet obligations under international law.

Mr Ratcliffe believes his wife is being held as “diplomatic leverage” by Iran.

The US authorities agree.

US Special Representative to Iran, Brian Hook, told Panorama Mrs Zaghari-Ratcliffe and other western detainees are effectively hostages.

“The Americans that are held in Iran are wrongfully detained and Iran has unfortunately a sad history going back to 1979 of hostage taking, when they took our American diplomats hostage,” he said.

“This is a practice. It’s a tool of statecraft.

“It’s part of Iran’s foreign policy to take people hostage who are innocent and then trade them later for some objective that they think advances their own objectives.”