Boris Johnson must be “brave” to secure the release of a British-Iranian woman jailed in Iran, her husband has said.
Richard Ratcliffe, who will meet the prime minister later, said he wanted to hold Mr Johnson to a promise he made in 2017 to “leave no stone unturned” to bring his wife home.
Nazanin Zaghari-Ratcliffe has been detained for almost four years over spying allegations she denies.
Her five-year-old daughter will also be at the meeting in No 10 on Thursday.
Mr Radcliffe last met Mr Johnson when he was foreign secretary in 2017.
That meeting came shortly after Mr Johnson had to apologise after wrongly suggesting that Mrs Zaghari-Ratcliffe had been training journalists at the time of her arrest.
She has always maintained she was in Iran visiting relatives.
During Thursday’s meeting, Mr Ratcliffe said he hoped to receive a credible sense from Mr Johnson that his wife’s case is a priority for him.
“I want to ask him to be brave,” he told BBC Breakfast.
Mr Ratcliffe said the prime minister needed to de-escalate tensions between the UK and Iran, amid the “tense situation” in the region following the US killing of top military leader Qasem Soleimani.
However, he added that the UK’s “soft” response to his wife’s case had “emboldened” Iran and the country needed to be “much tougher”.
“My message to him will be you can’t just wait for things to calm down, things are volatile inherently and they’ll get worse unless you do something clear,” Mr Ratcliffe added.
Kate Allen, director of Amnesty International UK, said the meeting with Mr Johnson was a “welcome step”.
She added: “Given recent deeply unsettling events in Iran, it’s now all the more important that Boris Johnson provides proper reassurances to the family that real and concerted efforts are being made at the highest levels to secure Nazanin’s release.”
The meeting comes as the Court of Appeal in London prepares to hear a case that can resolve a decade-long financial dispute between the UK and Iran.
The dispute is over the interest due on a debt owed by Britain to Tehran for an arms deal in the 1970s that was never fulfilled.
Mr Ratcliffe said his wife is a chess piece in the dispute, which he calls a festering sore in relations between the two countries – and he plans to ask Mr Johnson how he will solve it.
He has previously raised concerns that the recent escalation in tensions between the US and Iran could make matters worse for his wife.
Mrs Zaghari-Ratcliffe, 40, remains on medication for depression and on beta blockers – medicines which slow down the heart – for the panic attacks she’s been suffering in jail, her husband said.
The dual national has been detained since 2016, when she took her British-born daughter Gabriella to Iran to visit her parents. She was sentenced to five years in prison for spying.
Her family and the UK government has always maintained her innocence and she has been given diplomatic protection by the Foreign Office – meaning the case is treated as a formal, legal dispute between Britain and Iran.
While he was foreign secretary, Mr Johnson mistakenly said that Mrs Zaghari-Ratcliffe had been in Iran training journalists when she was arrested.
Four days later Mrs Zaghari-Ratcliffe was summoned to an unscheduled court hearing during which Mr Johnson’s comments were cited as proof she was engaged in “propaganda against the regime”.
Mr Ratcliffe previously said: “We’ve been quite clear in public that I feel he owes us and that he needs to do what he can to bring Nazanin home and to bring [home] the others held over similar reasons.”
On Tuesday, Mr Ratcliffe met the UK’s ambassador to Iran, Rob Macaire, who was himself arrested in Tehran – which Britain has condemned as illegal.
Mr Macaire was briefly held after attending an event advertised as a vigil for the 176 people who died in the crash of a Ukraine International Airlines Boeing 737 earlier this month.