Laywers for Nazanin Zaghari-Ratcliffe have asked the Defence Secretary for a meeting and called on the British Government to stand up to Iran for its “abusive treatment” of the jailed mother of one.

The lawyers want to meet Ben Wallace and his legal advisers to discuss the approach the British Government is taking on the issue of debt owed to Iran.

It has long been claimed that Mrs Zaghari-Ratcliffe – who is said to be in a perpetual state of anxiety – is being held in Iran in order to force the UK into settling a multimillion-pound dispute with the country.

In a letter to Mr Wallace, sent earlier this month, lawyers outline “grave concerns” about the UK’s “failure” to pay up.

“This failure continues to blight UK-Iranian relations and remains an intractable obstacle to Nazanin’s long-overdue release,” the letter said.

Lawyers also point out to Mr Wallace that they still have not received a proper response to a letter they sent to him almost a year ago.

“We wrote to you on this subject on 6 September 2019, but have yet to receive any substantive response, notwithstanding an indication that a response would be forthcoming,” lawyers said.

Mrs Zaghari-Ratcliffe’s husband Richard described the lack of response as “an astonishing lack of accountability”, adding that there has yet to be a response to the latest letter.

Requesting a meeting at the “earliest convenience”, the lawyers said: “There is still hope for an imminent positive ending for the Ratcliffe family, but this requires you to stand by your words of 2014 and take charge of the actions of IMS to find a way to honour the UK’s legal obligations to Iran (however unpalatable to the current US Administration).

“It also requires the UK Government to have the courage to stand up to Iran for its abusive treatment of Nazanin (and other innocent British nationals), and to protect their right to return to England and be reunited with their families.”

Mrs Zaghari-Ratcliffe was arrested at Tehran’s Imam Khomeini airport while travelling to show her young daughter, Gabriella, to her parents in April 2016.

She was sentenced to five years in prison over allegations, which she denies, of plotting to overthrow the Tehran government.

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

Mr Ratcliffe said Iran’s alleged use of prisoners as “collateral” over the multi-million pound dispute with the UK is “completely outrageous”.

He 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 his wife is a hostage in Iran and was on the brink of being freed from jail almost two and a half years ago.

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.

The debt to Iran 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.

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 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.

An MoD spokeswoman said: “The Defence Secretary’s position on this matter has not changed.

“As previous government statements have made clear, we remain committed to securing the immediate and permanent release of all arbitrarily detained dual British nationals in Iran and regularly lobby for their release at the highest levels.

“This includes through the Prime Minister, the Foreign Secretary and the British Ambassador in Tehran.”