LONDON: The husband of Nazanin Zaghari-Ratcliffe, the British-Iranian charity worker currently detained in Tehran, has said his wife is being held “hostage” by the regime.

Richard Ratcliffe added that she was close to being released in 2017, but that the deal fell through as she was being used as “diplomatic leverage” by Iran over a longstanding dispute dating back to the 1970s regarding the sale of 1,500 tanks by the UK to Tehran.

In a new documentary by the BBC’s “Panorama” program, it is alleged that prior to the Iranian revolution in 1979, the late shah paid £400 million ($525 million) for the tanks, but they were not delivered before his administration was overthrown.

The UK refused to deliver the tanks to the new regime, but also failed to return the money, leading to a longstanding rift between the two countries that is subject to ongoing legal battles.

The UK Foreign Office has said there is no link between Zaghari-Ratcliffe’s imprisonment and the dispute over the debt, calling such allegations “unhelpful.” Iran also denies the accusation.

But Brian Hook, the US special representative for Iran, told the “Panorama” documentary that such behavior by Tehran is commonplace.

“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.”

Zaghari-Ratcliffe, who worked as a project manager for the Thomson Reuters Foundation, has suggested that Iranian authorities had admitted to her that her release would depend on money from the failed tank deal being returned to Tehran.

Her husband told the “Good Morning Britain” program that his wife, who has been detained since April 2016 and was initially sentenced to five years in prison, could face a second court case once her sentence ends.

“It’s completely outrageous to be holding people and using them as collateral,” he said. “Behind closed doors the government will admit things — certainly previous ministers have been quite open with us.”

Zaghari-Ratcliffe was arrested in Iran at Imam Khomenei International Airport on April 3, 2016, while traveling to London, accompanied by her young daughter.

She was charged with “plotting to topple the Iranian government,” which she has always denied, and sentenced to five years in prison.

Earlier this year, she was temporarily released from Evin Prison, north of Tehran, to stay under house arrest with her parents as part of a furlough program to halt the spread of COVID-19 in the country.