Richard Ratcliffe, the husband of Nazanin Zaghari-Ratcliffe will personally urge Boris Johnson to pay a contested £400m debt the UK owes to Iran by providing the cash in humanitarian aid, so avoiding international sanctions.

Diplomatic sources have told the Guardian that Iranian and British officials have exchanged lists of humanitarian goods that could be bought for the UK for Iran, a development that suggests the proposal is under serious consideration. Iranian officials are said to be exploring the seriousness of the proposal.

The UK contends it cannot simply offset the debt through a direct financial payment since the Iranian defence ministry, to which the debt is owed, is subject to EU sanctions. The precise amount of the debt also remains in dispute.

The meeting in Downing Street between the two men on Thursday will be the first since Johnson became prime minister.

As foreign secretary, Johnson was accused by Ratcliffe of worsening his wife’s plight by wrongly stating she had been in Tehran at the time of her arrest to train journalists.

Zaghari-Ratcliffe has been held in a Tehran jail since her arrest in 2016 and she has been sentenced to five years in prison. Ratcliffe has warned that unless the British government takes proactive steps other dual nationals will be arrested and it is likely the Iranian authorities will lay further charges against her.

Timeline Imprisonment of Nazanin Zaghari-Ratcliffe in Iran Show Hide Arrest in Tehran Nazanin Zaghari-Ratcliffe is arrested at Imam Khomeini airport as she is trying to return to Britain after a holiday visiting family with her daughter, Gabriella. Release campaign begins Her husband, Richard Radcliffe, delivers a letter to David Cameron in 10 Downing Street, demanding the government do more for her release. Sentenced She is sentenced to five years in jail. Her husband says the exact charges are still being kept a secret. Hunger strike Nazanin Zaghari-Ratcliffe's health deteriorates after she spends several days on hunger strike in protest at her imprisonment. Appeal fails Iran’s supreme court upholds her conviction. Boris Johnson intervenes Boris Johnson, then Foreign Secretary, tells a parliamentary select committee "When we look at what [she] was doing, she was simply teaching people journalism". Four days after his comments, Zaghari-Ratcliffe is returned to court, where his statement is cited in evidence against her. Her employers, the Thomson Reuters Foundation, deny that she has ever trained journalists, and her family maintain she was in Iran on holiday. Johnson is eventually forced to apologise for the "distress and anguish" his comments cause the family. Health concerns Her husband reveals that Zaghari-Ratcliffe has fears for her health after lumps had been found in her breasts that required an ultrasound scan, and that she was now “on the verge of a nervous breakdown”. Hunt meets husband New Foreign Secretary Jeremy Hunt meets with Richard Ratcliffe, and pledges "We will do everything we can to bring her home." Temporary release She is granted a temporary three-day release from prison. Hunger strike Zaghari-Ratcliffe is on hunger strike again, in protest at the withdrawal of her medical care. Diplomatic protection The foreign secretary, Jeremy Hunt, takes the unusual step of granting her diplomatic protection – a move that raises her case from a consular matter to the level of a dispute between the two states. Travel warning The UK upgrades its travel advice to British-Iranian dual nationals, for the first time advising against all travel to Iran. The advice also urges Iranian nationals living in the UK to exercise caution if they decide to travel to Iran. Hunger strike in London Richard Ratcliffe joins his wife in a new hunger strike campaign. He fasts outside the Iranian embassy in London as she begins a third hunger strike protest in prison. Hunger strike ends Zaghari-Ratcliffe ends her hunger strike by eating some breakfast. Her husband also ends his strike outside the embassy.

Moved to mental health ward According to her husband, Zaghari-Ratcliffe was moved from Evin prison to the mental ward of Imam Khomeini hospital, where Iran’s Revolutionary Guards have prevented relatives from contacting her. Daughter returns to London Zaghari-Ratcliffe's five year old daughter Gabriella, who has lived with her grandparents in Tehran and regularly visited her mother in jail over the last three years, returns to London in order to start school. Temporary release Amid the threat of the coronavirus pandemic, she is temporarily released from prison, but will be required to wear an ankle brace and not move more than 300 metres from her parents’ home. New charges Iranian state media reports that she will appear in court to face new and unspecified charges. In the end, a weekend court appearance on a new charge of waging propaganda against the state that could leave her incarcerated for another 10 years is postponed without warning, leading Zaghari-Ratcliffe to say "People should not underestimate the level of stress. People tell me to calm down. You don’t understand what it is like. Nothing is calm."

It is acknowledged privately on both sides that there is a perceived connection between Zaghari-Ratcliffe’s arrest and the UK’s failure to pay the debt that goes back to 1979.

A further court of appeal hearing in London on Wednesday saw Iranian government lawyers urging the court to strike down a previous high court ruling that the UK need not pay interest on the debt for the period since EU sanctions were applied to the Iranian defence ministry in 2008. The interest is worth more than £20m.

Ratcliffe, seated in the public gallery, was placed one seat away from five Iranian officials monitoring the progress of the hearing.

Ahead of his meeting with Johnson, he told the Guardian he would press the prime minister to reveal whether he had a defined strategy to secure her release, and to argue that waiting for the crisis in relations to subside would only make the volatility of the crisis worse. “The UK government simply doing nothing will have consequences,” he warned.

He said he will ask Johnson whether he had explored the possibility of paying the debt through the provision of humanitarian goods, adding: “That is the obvious sanctions compliant route.”

He said the move would not strengthen the Iranian Revolutionary Guards since they would not receive any cash. He added: “The Iranians big up the point that they are being denied food and medicine and if we are trying to make a gesture that we are not spoiling for a fight it is a pretty obvious and clear path for the UK to take.”

The US has said foodstuffs and medicine are exempt from sanctions but has been slow to clarify how a possible Swiss channel could operate. Banks are wary of punitive US fines unless there is legal clarity.

Ratcliffe described the unpaid debt as “the elephant in the room” saying “all the time the debt is not being sorted, it is not a coincidence she is being held”. He said there should be no legal US objections to payment via humanitarian aid.

“The Iranians are clearly signalling that if the Brits are decent they will pay this money and ergo if not there will be consequences. My sense is that the Iranians will keep Nazanin and at the end of her sentence will charge her again. There is also a very real risk that more Brits will be picked up as part of this wider narrative to blame an outsider for what is happening in Iran.”

He insisted he would not retract any of the tough things he had said to the prime minister in recent months. “I meant it and we will see how he reacts. I am not planning to apologise. I am responsible for what I have said for better or worse. I am a great believer in transparency and honesty. It may come back to haunt me, so be it.”

He added he will tell Johnson “you do need to find a way to protect the people that are being held and that means you have to be brave. You have to be clear in your priority in protecting people. Don’t think waiting does that. The British government has an instinct to do no harm, but it cannot in this context. You have to be clearer about what is off limits.”