Foreign Secretary Boris Johnson will visit Iran this weekend to try to secure the release of jailed British mother Nazanin Zaghari-Ratcliffe.
Mr Johnson’s trip to Tehran is only the third by a UK Foreign Secretary since 2003 and comes at a time of tension in the Middle East over Donald Trump’s announcement that he is recognising Jerusalem as the capital of Israel.
In wide-ranging talks with Iranian foreign minister Mohammad Javad Zarif, Mr Johnson will seek to shore up bilateral relations and urge Tehran to stick by the terms of its 2015 nuclear deal.
Mrs Zaghari-Ratcliffe is serving a five-year sentence over allegations, which she denies, of plotting to overthrow the Tehran government.
She was arrested last year during a holiday visit to show her baby daughter Gabriella to her parents.
Reports suggest she could appear in court again on Sunday.
There have been threats to increase her sentence by five years following Mr Johnson’s gaffe of telling a parliamentary committee that she had been in Iran to train journalists. He later acknowledged this was not the case.
It is understood that Nazanin’s husband Richard will not accompany Mr Johnson after receiving advice that it may not help his chances of seeing his wife in prison.
Tehran does not recognise Mrs Zaghari-Ratcliffe’s dual UK-Iranian nationality and refuses access to her for representatives of the British authorities, making a prison visit for the Foreign Secretary unlikely during the trip.
Hers is one of a small number of cases of dual nationals whose release Britain is seeking on humanitarian grounds on which Mr Johnson will push for progress.
The Foreign Office has declined to name the other individuals involved - or even identify the number in jail - after their families asked for their cases to be kept out of the public eye.
Relations with Iran have been strained in recent years despite the reopening of the UK embassy in Tehran in 2015, but London has detected possible signs of greater openness to dialogue in recent months.
Diplomats are hopeful that Mr Johnson’s visit will help establish a “load-bearing” relationship, allowing them to work with one another seriously on issues of concern.
It is thought that Iran appreciated the UK’s firm response to Mr Trump’s decision last month not to recertify the 2015 deal under which sanctions were lifted in return for an end to its military nuclear programme.
Mr Johnson travelled to Washington to urge Congress not to tear up the deal and made clear that Britain remains convinced it is essential.
Despite disgruntlement in Tehran at the limited scale of trade and banking access opened up by the deal, Mr Johnson will seek to persuade Mr Zarif that it has nonetheless been good for Iran’s economy as well as for regional stability.
He will reassure him of the continued support of the UK, alongside other European sponsors, for the deal and press him to ensure continued co-operation with international inspection teams from the IAEA.
In stark contrast to Mr Trump, the UK believes Iran’s compliance with its side of the deal has been largely good.
He is expected to raise concerns over allegations - denied by Tehran - that Iran has supplied missiles to Houthi rebels in Yemen, one of which is believed to have been used in the recent attack on an international airport in Saudi capital Riyadh.
And he is certain to hear complaints about the difficulties which Iranian authorities and banks have had in accessing the banking network in the UK, where many institutions with close links to the US are wary of taking on business that may lay them open to charges of breaching American sanctions.
Iran is also likely to raise the issue of a £400 million debt, dating back to an abortive order for tanks by the pre-revolutionary regime in 1979, which remains unpaid despite a court ruling against the UK.
The cash has already been deposited in a court for payment and it is understood that work is underway to find a sanctions-compliant method for handing it over.
The Foreign Office has always stressed that the payment of the money is in no way linked to the cases of Mrs Zaghari-Ratcliffe and the other imprisoned dual nationals.
Tomorrow’s visit to Iran forms part of a three-day trip to the Middle East.
Mr Johnson is also taking in Oman today and the United Arab Emirates on Sunday, in which moves to end humanitarian suffering in war-torn Yemen will be high on the agenda.
A Foreign Office spokesman said: “This visit comes at a crucial time for the Gulf region and provides an opportunity to discuss a peaceful solution to the conflict in Yemen, the future of the Iran nuclear deal and the current volatility in the Middle East.
“This is the first visit of the Foreign Secretary to Iran and we expect talks to cover a wide range of issues from the bilateral relationship to regional security.
“The Government remains very concerned about all our dual nationals detained in Iran and has been doing everything it can to make progress on their cases, while approaching them in a way that we judge is in their best interests.
“The Foreign Secretary will urge the Iranians to release dual nationals where there are humanitarian grounds to do so.”