The husband of the detained British-Iranian aid worker Nazanin Zaghari-Ratcliffe is beginning a hunger strike in Whitehall, demanding the government does more to secure her release.
Mrs Zaghari-Ratcliffe, who has been held in Iran for five years on spying charges, recently lost her appeal against a second prison sentence.
Richard Ratcliffe said his wife was "increasingly distraught".
The Foreign Office says it will "continue to press Iran" on the issue.
Mrs Zaghari-Ratcliffe, a 43-year-old mother-of-one from London, has been detained in Iran since 2016 and has not seen her daughter for two years.
She has been serving the second of two prison sentences, this one on parole for a conviction of propaganda against the Iranian regime. She is staying with her mother in Iran - but is not allowed to leave the country.
Mrs Zaghari-Ratcliffe has always denied any wrongdoing.
But now she faces a return to prison, after losing an appeal against the most recent sentence. Mr Ratcliffe said it was only a matter of time before she would be summoned back to jail.
The hunger strike began on Sunday near to the Foreign Office and Downing Street in London. It is the second time Mr Ratcliffe has used the tactic, after a 15-day hunger strike outside the Iranian embassy in London in 2019.
"Two years ago I went on hunger strike in front of the Iranian embassy, on the eve of Boris Johnson taking over as prime minister," said Mr Ratcliffe in a statement online.
"We are now giving the UK government the same treatment. In truth, I never expected to have to do a hunger strike twice. It is not a normal act. It seems extraordinary the need to adopt the same tactics to persuade government here, to cut through the accountability gap."
He said that although Iran remained the main country responsible, "the UK is also letting us down".
"It is increasingly clear that Nazanin's case could have been solved many months ago - but for other diplomatic agendas. The PM needs to take responsibility for that."
He added: "It can be difficult to capture the feeling of a life wasting away, watching prison creep closer while we sit in the PM's in-tray."
Mr Ratcliffe said he was making four demands from Mr Johnson, including recognising Mrs Zaghari-Ratcliffe as a hostage, and for the UK to push for an end to hostage-taking when negotiating the Iran nuclear deal.
He also called for the government to pay the £400m debt that the UK owes Iran, dating back from a deal between the two sides over tanks in the 1970s.
Mr Ratcliffe believes his wife has been imprisoned as leverage for the debt.
He spoke to the new Foreign Secretary Liz Truss earlier this month, but said he was told the government's response was to do nothing yet until Mrs Zaghari-Ratcliffe was returned to prison.
"For us, reimprisonment is too late, it would mean not seeing Nazanin until 2023," he said.
The MP Tulip Siddiq - who represents the constituency where the Zaghari-Ratcliffes live - called on the government to listen to Mr Ratcliffe.
"It breaks my heart that my constituent Richard Ratcliffe has once again been forced to go on hunger strike to protest against the government's failure to free Nazanin," she said.
"It should never have come to this. It's time for the government to listen to the demands of Nazanin's family, including paying the debt we owe to Iran, and finally bring her home."
And the boss of charity Amnesty International called the situation "incredibly upsetting".
"Like Richard, we've grown tired of hearing ministers saying they're 'doing all they can' for Nazanin and other arbitrarily-detained Britons in Iran - it doesn't look like that to us, and it certainly hasn't produced results," said Sacha Deshmukh.
He demanded the government sets out a strategy for getting Mrs Zaghari-Ratcliffe home, and added: "We call on Boris Johnson, Liz Truss and other ministers to take the time to come out of their offices to visit Richard at his tent. Ministers need to hear first-hand how desperate this situation is."
On Sunday, a Foreign Office spokesperson said: "Iran's decision to proceed with these baseless charges against Nazanin Zaghari-Ratcliffe is an appalling continuation of the cruel ordeal she is going through.
"Instead of threatening to return Nazanin to prison, Iran must release her permanently so she can return home.
"We are doing all we can to help Nazanin get home to her young daughter and family and we will continue to press Iran on this point."