Iran’s alleged use of prisoners as “collateral” over a multi-million pound dispute with the UK is “completely outrageous”, the husband of Nazanin Zaghari-Ratcliffe has said.

Richard Ratcliffe said there was “clearly a stand-off” between the two countries and that he feared his wife could face a second court case once her sentence comes to an end next spring.

His comments come as a new BBC Panorama investigation claims the British-Iranian mother-of-one is a hostage in Iran and was on the brink of being freed from jail almost two and a half years ago.

I think it's fair that neither government likes to be too honest about what is going on

Mr Ratcliffe told the documentary, which looks at the detention of western citizens in the country, that the family had been given a date of December 28 2017 for his wife’s return to the UK.

But it is unclear why the deal fell through.

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.

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.

Mr Ratcliffe appeared on ITV’s Good Morning Britain (GMB) on Monday and was asked about the UK Government’s denial of a link between his wife’s detention in Iran and claims of a debt between the two countries.

He told the show: “Well of course there should be no link.

“It’s completely outrageous to be holding people and using them as collateral.

“Behind closed doors the Government will admit things, certainly previous ministers have been quite open with us.

“And yes, I think it’s fair that neither government likes to be too honest about what is going on.”

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.

Mr Ratcliffe told GMB that his wife would be “counting down the months” until the end of her sentence.

But he added: “I think, behind closed doors, they keep saying there’s a second court case, they keep talking about running it.

“My fear is that’s what happens.”

Mrs Zaghari-Ratcliffe 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.

She 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.

The BBC Panorama documentary airs on Monday night on BBC One at 7.30pm.