Video report by ITV News Political Correspondent Libby Wiener
Nazanin Zaghari-Ratcliffe's husband has said his wife is "being used as a chess piece" and urges the government to be "tougher with Iran" after his meeting with Boris Johnson.
Richard Ratcliffe spoke to reporters on Thursday, as he seeks to increase the pressure to free his wife, who has been detained in Tehran since 2016, saying: "In my sense, the government needs to be tougher with Iran."
The meeting comes as fears have increased over Ms Zaghari-Ratcliffe's chances of freedom after the US killed top Iranian general Qassem Soleimani, causing spiralling tensions in the Middle East.
When asked what his message was to Tehran, he said: "Our story has gone on for four years, we're going to be a thorn in their side."
"They know that they are playing a game of chess with an ordinary person's life, I hope they realise at some point it's the wrong way to go," he added.
Richard Ratcliffe walking in to Number 10 with his daughter Gabriella
Mr Ratcliffe said he came away from meeting with the prime minister and Foreign Secretary Dominic Raab feeling that they were both very committed and cared.
However he added: "I don't stand here hopeful to be honest, my job is to keep going and keep pushing."
He added that his five-year-old daughter Gabriella met Number 10 cat Larry and that she was gifted with a toy cat.
Ms Zaghari-Ratcliffe, a 40-year-old mother from London, is serving a five-year sentence after being arrested during a holiday with her daughter and accused of spying.
Her family and the UK Government have always maintained her innocence and she has been given diplomatic protection by the Foreign Office.
Diplomatic protection is a little-used mechanism by which the Government can try to help individuals it believes have been wronged by another state.
It means Ms Zaghari-Ratcliffe's treatment by Iran is a formal state issue and recognises that the legal proceedings brought against her failed to meet international standards.
Mr Johnson has been persistently criticised for wrongly claiming, when he was foreign secretary, that she was training journalists at the time of her arrest.
Four days later she 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".
Ms Zaghari-Ratcliffe is among as many as five people with dual British-Iranian nationality, or with UK connections, believed to be in prison in Iran.
British Ambassador Robert Macaire returned to London for talks after being labelled "persona non grata" by Iran's judiciary, and hardline protesters burned an effigy of the diplomat.
He had been arrested and briefly detained after attending a vigil for the 176 people, including four Britons, who were killed when Iran accidentally downed a Ukrainian jet amid spiralling tension.
It is acknowledged privately on both sides there is a perceived connection between Zaghari-Ratcliffe's arrest and a long-running dispute which goes back to 1979.
There are some hopes that diplomatic tensions could ease between London and Tehran if the dispute, about the UK's failure to pay the £400m debt, is settled in the Court of Appeal this week.
Labour MP for West Hampstead and Kilburn, Tulip Siqqid, said on social media she was "absolutely gobsmacked" the Government still denies the £400m debt.
Iran's ambassador to the UK, Hamid Baeidinejad, tweeted if a portion of the debt is assigned to Tehran then it will mean "the legal process of the case is coming to an end and there will be no excuse for default" from the Government.
The sum has been outstanding since pre-revolutionary Iran paid the UK for 1,500 Chieftain tanks in the 1970s.
The deal was cancelled after the Shah of Iran was deposed in 1979, but Britain has refused to heed Iran's demands to hand back the money.