A British-Iranian mother detained in Iran has begun a new hunger strike.
Nazanin Zaghari-Ratcliffe was jailed for five years in 2016 after being convicted of spying, which she denies.
Her husband, Richard Ratcliffe, who is joining her in refusing food, said she wanted her unconditional release.
It comes amid growing tensions between the UK and Iran, after Britain said the Iranian regime was "almost certainly" responsible for attacks on two oil tankers in the Gulf of Oman.
Iran has denied being behind Thursday's explosions but the UK Foreign Office said "no other state or non-state actor could plausibly have been responsible".
Foreign Secretary Jeremy Hunt urged Iran to "do the right thing" and release Mrs Zaghari-Ratcliffe.
"Our message to Iran is whatever the disagreements you may have with the United Kingdom, there is an innocent woman at the heart of this," he said.
With Richard Ratcliffe this morning whose family have been separated for over 3 years. As Richard continues his campaign for Nazanin’s release at the Iranian embassy,my message to Iran: DO THE RIGHT THING, SHOW THE WORLD YOUR HUMANITY & LET THIS INNOCENT WOMAN HOME #freenazanin pic.twitter.com/bitSctNmn9— Jeremy Hunt (@Jeremy_Hunt) June 15, 2019
Mr Ratcliffe said he had received a phone call from his wife to tell him she had informed the Iranian judiciary that she had begun a hunger strike - although she would still drink water - to protest against her "unfair imprisonment".
He added that his wife sounded "nervous but calm".
"Her demand from the strike, she said, is for unconditional release.
"She has long been eligible for it. I do not know the response from the Iranian authorities," he said.
He said his wife had made the decision following the fifth birthday of their daughter, Gabriella.
Gabriella has not been allowed to leave Iran following her mother's arrest and is living with her maternal grandparents.
"Nazanin had vowed that if we passed Gabriella's fifth birthday with her still inside, then she would do something - to mark to both governments - that enough is enough," Mr Ratcliffe said.
"This really has gone on too long."
Friends and family gathered outside the Iranian embassy in London on Saturday, singing Happy Birthday via a video call to Gabriella, who celebrated her birthday on 11 June, and sharing a unicorn-shaped birthday cake.
As he began his own hunger strike, Mr Ratcliffe said: "I said that if she did it again I would stand in solidarity with her.
"A hunger strike in prison, nobody gets to see it - a hunger strike here is much more public. I will keep her story public."
The next UK prime minister should make it their top priority to "protect British citizens from unfair imprisonment, from torture", said Mr Ratcliffe.
Mr Ratcliffe has urged the Iranian authorities to allow British embassy officials to visit her to check on her health during her hunger strike.
He said that if she was not freed within the next few weeks, he wanted the Iranians to grant a visa so he could visit her himself.
The 40-year-old aid worker was arrested at Tehran's Imam Khomeini Airport in April 2016 and has always maintained the visit was to introduce her daughter, Gabriella, to her relatives.
She is serving a five-year sentence in Tehran's Evin Prison.
Mr Hunt granted Mrs Zaghari-Ratcliffe diplomatic protection in March, but Tehran refuses to acknowledge her dual nationality.
Amnesty International UK's director Kate Allen said her plight was "truly heart-breaking".
"Nazanin is a prisoner of conscience, unfairly jailed after a sham trial and subjected to all manner of torments - including months in solitary confinement and endless game-playing over whether she would receive vital medical care," she said.