The UK has long said that it would pay back the money but continues to insist there is no link between that payment and discussions about the release of Ms Zaghari-Ratcliffe. Her lawyer, Hojjat Kermani, said on Tuesday he believed his client’s case was linked to talks about the collapsed Iran nuclear deal happening in Vienna. 

Iran is under some economic sanctions after the original 2015 deal, in which they agreed not to pursue nuclear weapons, was scuppered. But there are hopes it could be revived.

If a deal can be struck then Iran, a major oil-producing nation, could be released from sanctions, potentially helping stabilise prices as the West scales back Russian oil imports following the Ukraine invasion.

‘Behind the scenes’

Mr Kermani said: “In reality, her release is very much tied to the outcome of the nuclear negotiations which are still going on behind the scenes. I have no idea how the issue of the UK’s debt to Iran is affecting the talks in Vienna.”

But a senior government source pushed back on the suggestion the two issues were linked, saying talks on Mrs Zaghari-Ratcliff were going on for months before the invasion.

Mrs Zaghari-Ratcliffe, a project manager with the Thomson Reuters Foundation, was arrested at a Tehran airport in April 2016.

She was later convicted of plotting to overthrow the clerical establishment. Her family and employer deny the charge as baseless.

Tulip Siddiq, the Labour MP who represents Mrs Zaghari-Ratcliffe’s constituency, said: “I am very pleased to say that Nazanin Zaghari-Ratcliffe has been given her British passport back.

“She is still at her family home in Tehran. I also understand that there is a British negotiating team in Tehran right now.”

Mr Kermani told The Telegraph that he hoped Mrs Zaghari-Ratcliffe would be released “within days”.

“We hope this will happen sooner rather than later as it has been a long wait,” he said.

The release of Mrs Zaghari-Ratcliffe and another detained dual national, Anoosheh Ashoori, has been tied to the debt of about £400 million that Britain owed Iran over the Chieftain tanks.

Avoids breaching sanctions

While neither Iran nor the UK have explicitly linked the repayment of the debt to the release of dual nationals, their families and lawyers believe this is the agreement being negotiated.

Britain has acknowledged it owes the money and has been in negotiations with Iran over repayment, with the Government saying it must be done in a way that avoids breaching sanctions on Iran.

On Tuesday Sara Massoumi, a prominent Iranian journalist, reported on rumours that the debt had already been repaid but the Foreign Office would not confirm this.

“We continue to explore options to resolve this case and will not comment further,” a Foreign Office spokesman said.

“We have long called for the release of unfairly detained British nationals in Iran. We don’t comment on speculation.”

Asked about the case, Boris Johnson said: “I don’t want to tempt fate.

“Negotiations about all our difficult consular cases have been going on for a long time,” he told reporters, saying “quite delicate discussions” are still taking place”.

On Tuesday, a Number 10 official said of the debt: “There is no change on our position to that.

“We are committed to paying the debt. We’re exploring options to resolve it – it has not been resolved.”