Britain is preparing to pay Iran a £450million debt to help free a British mother jailed there for alleged espionage, it was reported last night.

Boris Johnson wants to improve relations with Iran as he battles to free Nazanin Zaghari-Ratcliffe who is locked in a Tehran jail.

He has ordered officials to find a way to get around UN sanctions which have stopped the UK paying Iran a debt over a 1970s arms deal.

The Shah of Iran paid Britain £650million for 1,750 Chieftain tanks but only 185 had been delivered when he was toppled in 1979 and the new government cancelled the order.

Britain was told to pay back £450million by the International Chamber of Commerce in a 2009 ruling but sanctions on military equipment prevent payment.

Boris Johnson has finally met the husband of jailed British mother Nazanin Zaghari-Ratcliffe on Wednesday, Richard Ratcliffe (pictured together)

Richard Ratcliffe is meeting Boris Johnson about securing the release of his wife Nazanin Zaghari-Ratcliffe, pictured with their daughter Gabriella, who is also 'suffering' badly

Zaghari-Ratcliffe's husband Richard, who yesterday met Boris Johnson, told the Sun: 'It is important that the UK honours its international legal obligations, so that Iran can honour its legal obligations.

'They are separate things, but it is good for the atmosphere if they are all served.'

A diplomatic source told The Telegraph: 'Building a strong relationship and trust is the key to securing her safe return.'

Two senior government sources said they fear Iran is holding Mrs Zaghari-Ratcliffe as 'collateral' for the debt.

The government has insisted the debt repayment and the release of Ms Zaghari-Ratcliffe are not connected.

A government spokeswoman said: 'This is a long standing case and relates to contracts signed over 40 years ago with the pre-revolution Iranian regime.

'Funding to settle the debt was paid to the high court by the Treasury in 2002. Iran's Ministry of Defence remains subject to EU sanctions.

'It is wrong to link a completely separate debt issue with any other aspect of our bilateral relationship with Iran.'

King Hussein of Jordan drives a Chieftain tank at Bovington, Dorset in 1966

Mr Ratcliffe yesterday urged Boris Johnson to abandon his 'softly softly' approach with Iran to secure his wife's release.

Richard Ratcliffe said his hour-long meeting with the Foreign Secretary was 'positive and constructive' and he hoped his wife and daughter would be home by Christmas.

He said his wife was 'on the verge of a nervous breakdown' and her mental state had suffered since a gaffe by Mr Johnson about her activities in Iran led to the threat of further charges.

Mr Johnson is 'keen' to take Mr Ratcliffe to Iran so he can see his wife and child for the first time in almost two years, it was revealed.

But the Foreign Secretary did not grant his request for diplomatic protection for his wife - for now - because he fears it could damage relations with Tehran.

Mr Ratcliffe challenged him afterwards and said: 'It's important that where softly-softly doesn't work, where it has been escalated by the past couple of weeks' events and the Foreign Secretary's words being used, I think it is appropriate'.

Mr Ratcliffe wants the Foreign Secretary to take him to Iran and guarantee him 'safe passage' so he can see his wife and daughter Gabriella for the first time in 19 months

Mr Ratcliffe, pictured with his local MP Tulip Siddiq, right, who said their meeting was 'positive and constructive'

Mr Ratcliffe, whose wife was arrested on spy charges last year while on holiday and jailed for five years, urged him to escalate the Government's support for her as the 'softly-softly' approach was not working.

But he described yesterday's meeting - their first face-to-face - as 'positive and constructive'.

Mr Johnson promised 'no stone will be unturned' in pursuing her rescue from an Iranian prison and he will work with Tehran to secure 'safe passage' to the country, which has blocked his entry for 19 months.

Mr Ratcliffe told a press conference after their first ever meeting that he still believes he could bring his wife and daughter Gabriella home to London in time for Christmas.

He also thanked the 1.2million people who have signed a petition calling for Theresa May to intervene and said his wife 'appreciated' their support.

Yesterday he told the Foreign Secretary to take him to Iran on his official trip later this year and guarantee him 'safe passage' because Iran will not give him a visa.

He has argued she must be given diplomatic protection - used by governments to rescue citizens being badly treated by a foreign power.

Mr Johnson said Britain has to consider if the move 'will have a positive impact' - but agreed that Foreign Office lawyers should meet with Nazanin's team to discuss it further.

The Foreign Secretary (pictured running acriss Whitehall yesterday) has faced calls to resign over claims that Nazanin Zaghari-Ratcliffe could have her jail time increased

Mr Ratcliffe said: 'We talked, of course, about the point of diplomatic protection ... and it's different from diplomatic immunity.

'Diplomatic protection is in essence when a state like Britain decides that Nazanin was being treated badly because she is British and she is entitled to be protected as an extension of the British state. It is not unprecedented, but it is a big step.

'I said I thought it would be important and helpful. The Foreign Secretary and the Foreign Office expressed reservations, and we agreed that there are some questions that we have sent from the lawyers.

'They have agreed to answer the questions and then for the lawyers to sit down and talk it through. Both legally and then also practically.

'But certainly, I think it is an important thing for us to be pushing for.'

The pair have spoken on the phone but never met in person, until yesterday.

Mr Johnson apologised on Monday for causing her 'anguish and distress' as he admitted making mistakes in his handling of her case.

Ahead of the meeting, Mr Johnson had repeated his promise that he would 'leave no stone unturned' in efforts to secure Mrs Zaghari-Ratcliffe's release.

He said: 'I can tell you that people here in the Foreign Office and across government have been working very hard over the last 19 months to secure the release of Nazanin Zaghari-Ratcliffe and indeed to solve some other very difficult consular cases in Iran.

'And we're going to continue to do that and we will leave absolutely no stone unturned in our efforts to do that.'

Richard Ratcliffe arrives at the Foreign Office for his first face-to-face meeting with Boris Johnson over his wife Nazanin's detention in Iran

Nazanin Zaghari-Ratcliffe was visiting her parents in Iran with her young daughter Gabriella when she was arrested by the Revolutionary Guard in Tehran Airport

Foreign Secretary Boris Johnson has apologised and said: 'My remarks should have been clearer' after his gaffe wrongly suggested she was in Iran training journalists not on holiday

The Foreign Secretary has faced calls to resign over claims that Nazanin Zaghari-Ratcliffe could have her jail time increased after he wrongly said she had gone to the country to train journalists.

But Labour withdrew its demands for him to quit after her husband warned this would not help her case.

20 female Iranian inmates step in to support bereft Briton arrested on holiday Iranian female political prisoners and human rights activists in the same jail as Nazanin Zaghari-Ratcliffe have stepped in to support her, it was revealed. The mother-of-one is said to be on the verge of a breakdown after being arrested while on holiday visiting her parents with her daughter. Her husband Richard said women including Narges Mohammadi in the Evin jail (pictured), which is known as 'Evin University' due to the number of academics, journalists and other professionals housed there, are supporting her. Mr Ratcliffe told the Evening Standard: 'A few of [the women] have been there for a very long time and make sure that everyone is kept busy. 'The easiest thing to do is to lie on your mattress feeling sorry for yourself. They sat her down and said why don't you teach English to the other prisoners. So she does that once a week. 'It's a way that everybody can contribute. They have a cleaning rota, they do craft work. This is experienced women saying 'we will be strong' and 'stand up for yourselves'. 'I am thankful for all they have done for Nazanin to remind her of the kindness in Iranian culture, and the hope to survive.' Advertisement

Asked what reservations Mr Johnson had expressed about granting diplomatic protection status, Mr Ratcliffe said: 'He didn't personally give a long list of objections.

'He asked how it would help, in a nutshell. What did we think doing it would make different from what we are doing currently?

'I said, 'I'm not a lawyer, I think it would help, I think it would send an important signal that the way Nazanin is being treated is unacceptable.

'We agreed to keep talking about it.'

Mr Ratcliffe expressed concern for his wife's state of health.

'She talks about being on the verge of a nervous breakdown. I absolutely believe that's true. I think it's important I don't exaggerate anything in the media and I'm not melodramatic, but she is in a difficult place.'

Ms Siddiq, who is Mrs Zaghari-Ratcliffe's MP, described the meeting as 'encouraging'.

The MP said Mr Ratcliffe had failed to obtain a visa to visit Iran over the last 19 months, and had not seen his daughter Gabriella, who is living in the country with her maternal grandparents, during that time.

'Gabriella has lost the ability to speak English now, so it is very difficult for Richard and Gabriella to communicate.

'Their communication is via Skype, but as she doesn't speak English and he doesn't speak Farsi, you can imagine it's not an ideal solution.

'So, if he gets to go with the Foreign Secretary, he gets to see his daughter for the first time in 19 months. And if he's there, he has the right to visit Nazanin in prison as her relative.'

Ms Siddiq said Mr Johnson had made clear he would 'certainly' push to see Mrs Zaghari-Ratcliffe himself in prison if he goes to Iran.

In the past, British diplomats have been to the prison but been unable to see her, she said.

'I want to make sure that if he goes there, he sees my constituent face-to-face,' said Ms Siddiq. 'Otherwise it's a wasted trip.'

Mr Johnson said he 'did not want to rule out' the option of granting diplomatic protection status, as Mrs Zaghari-Ratcliffe's supporters have been asking for some time, said the MP.

Ms Siddiq added: 'The Foreign Secretary categorically said he would leave no stones unturned in this campaign to get my constituent home, and that's what we are going to hold him to.'

Mrs Zaghari-Ratcliffe is pictured with her husband Richard who is desperate to see her back home with their three-year-old for Christmas

Mrs Zaghari-Ratcliffe, 38, was arrested in Tehran in April last year and sentenced to five years' imprisonment for working against the Iranian regime, a charge she denies.

Her husband has campaigned for her release, insisting she was on holiday with their 18-month-old daughter Gabriella so the child could meet her grandparents in Iran.

Despite her family pleading for help from the Foreign Office for more than 18 months, Mr Johnson told the foreign affairs committee earlier this month she had been training journalists in Iran.

The Iranian state broadcaster said this amounted to an 'unintended admission' of her guilt.

Mr Ratcliffe said his wife is 'on the verge of a nervous breakdown' and angry at the Foreign Secretary for allowing the situation to become 'a shambles'.

He said her mood was 'uncontrollable' and revealed that she had experienced pain in her breasts and been taken to hospital for an ultrasound, which found lumps. The mother-of-one has a family history of breast cancer and a former cellmate described seeing her hair falling out in 'huge clumps'.

Mrs Zaghari-Ratcliffe, 38, was seen by a specialist in Tehran on Saturday. The consultant said he thought the lumps were likely to be benign but added that some cancers were linked to stress, her husband said.