Johnson will seek the release of Nazanin Zaghari-Ratcliffe, who was sentenced to five years in prison in September 2016...
Johnson will seek the release of Nazanin Zaghari-Ratcliffe, who was sentenced to five years in prison in September 2016 for taking part in mass anti-regime protests in 2009 -- a charge she denies.
She is due to face fresh propaganda charges in court on Sunday, which could land her several more years in jail.
Johnson will meet with his Iranian counterpart Javad Zarif and will also hold talks on the Iranian nuclear deal.
Johnson was to arrive in Muscat on Friday for the start of a three-day visit to Oman, the United Arab Emirates and Iran.
"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," a Foreign Office spokesman said.
The trip will be Johnson's first visit to Iran, the first of a British foreign secretary since 2015 and only the third since 2003.
"We expect talks to cover a wide range of issues from the bilateral relationship to regional security," the spokesman said.
"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."
Tehran does not recognise Zaghari-Ratcliffe's dual nationality, and refuses access to her for representatives of the British authorities, making a prison visit for Johnson unlikely.
Zaghari-Ratcliffe was working at the Thomson Reuters Foundation (TRF) -- the media organisation's philanthropic arm -- at the time of her arrest.
She is due to face fresh charges in court of "spreading propaganda" on Sunday.
TRF chief executive Monique Villa said recently that the new charges risked a further 16-year prison sentence and were "a mockery of justice".
Zaghari-Ratcliffe was arrested at Tehran airport on April 3, 2016 after visiting family. She was travelling with her daughter Gabriella, now three years old.
The case has become highly politicised, especially after a "slip of the tongue" by Johnson last month when he stated that Zaghari-Ratcliffe had been training journalists in Iran.
Zaghari-Ratcliffe's husband Richard Ratcliffe said Johnson's visit to Iran is likely to give his wife hope.
"Nazanin has a new court case on Sunday, so it is really, really great that he is able to be there, just to press how important it is that she could be home with her family," he told Sky News television.
"Him being there can only make things better. It makes a clear statement that he is concerned about Nazanin, he is concerned about the other cases.
"I am sure the fact that the foreign secretary is going will have given her some hope."