UK Foreign Secretary Liz Truss said she was "absolutely determined to secure the release of detained British nationals" in Iran, after she met Tehran's Foreign Minister, Hossein Amir-Abdollahian, on Wednesday.
"The Foreign Secretary held her first meeting with Iranian Foreign Minister Hossein Amir-Abdollahian to discuss bilateral, nuclear and regional issues," the Foreign, Commonwealth and Development Office said.
"She called for the release and return home of British dual national detainees. She urged Iran to return rapidly to the [2015 nuclear deal with world powers] negotiations in Vienna with a view to all sides coming back into compliance and reducing tensions over Iran’s nuclear programme."
The husband of British-Iranian detainee Nazanin Zaghari-Ratcliffe earlier urged Ms Truss to impress on Iran that taking hostages is an "unacceptable practice".
Ms Truss on Thursday issued a statement marking a milestone in Mrs Zaghari-Ratcliffe's incarceration. “Today marks 2,000 days since Nazanin Zaghari-Ratcliffe’s cruel separation from her family," she said. “She is going through an appalling ordeal. We are working tirelessly to secure her return home to her family.
“I pressed the Iranian foreign minister on this yesterday and will continue to press until she returns home.”
Thursday marks 2,000 days since Ms Zaghari-Ratcliffe was detained, and her MP Tulip Siddiq, along with more than 175 other parliamentarians, accused the government in a letter of complacency in its response to Tehran.
The MPs and peers urged Mr Johnson to call her and other dual nationals hostages, and resolve a long-standing legal row with Iran over a £400 million ($544.8m) debt linked to a 1970s arms deal.
“After 2,000 days of disappointments and let-downs, I do sometimes wonder what will get the government to bring Nazanin and the others home," said Richard Ratcliffe, Nazanin’s husband.
“What will end the years of drift or get the prime minister to deliver on his word?”
Mr Ratcliffe said the signatories to the letter had called for the government to “get tough with Iran’s hostage-taking, and grow a backbone in how it protects British citizens held overseas”.
“The refusal to acknowledge this as hostage-taking, or to impose any cost on Iran for the hostage-taking of British citizens, enables the practice to continue,” the MPs and peers said in their letter, due to be delivered to Downing Street on Thursday.
They accused the government of failing to resolve the debt dispute with Tehran.
"It is shameful that the sins of the British government should be allowed to sit for so long on the shoulders of ordinary, innocent citizens and their families."
The letter will be delivered by Mr Ratcliffe and his daughter Gabriella, along with Ms Siddiq, the Labour member for Hampstead and Kilburn.
Fellow Labour MP Janet Daby and Elika Ashoori, daughter of detainee Anoosheh Ashoori, will also be at Downing Street.
“The time for excuses is over," Ms Siddiq said. "This cross-party letter sends a direct message to the prime minister that a new approach is needed if we are going to free Nazanin and end this barbaric hostage-taking.”
Mr Ratcliffe will stand on a giant snakes and ladders board in Westminster on Thursday to symbolise the feeling of being “caught in a game between two governments”, said Amnesty International, which is supporting his campaign.
“People’s lives shouldn’t be treated like a game by governments," said Amnesty’s UK chief executive, Sacha Deshmukh.
"It’s excruciating to see how Nazanin and her family are being made to suffer like this. We’re hoping Liz Truss will make the job of securing Nazanin’s release a major priority as foreign secretary.”
Ms Zaghari-Ratcliffe is one of several people with British or dual-British nationality detained in Iran.
She was held in Tehran in 2016 on national security charges while taking her daughter Gabriella to see her family.