A British-Iranian mother detained in Tehran has begun a three-day hunger strike in protest against being denied specialist medical care.
Nazanin Zaghari-Ratcliffe was jailed for five years in 2016 after being convicted of spying, which she denies.
The aid worker has endured pressure to spy on the UK for Iran, according to her husband, Richard Ratcliffe.
Foreign Secretary Jeremy Hunt has summoned the Iranian ambassador to the UK to discuss the case.
Mr Hunt has praised her bravery and criticised Iran for - in his words - allowing an innocent mother to feel she needs to resort to a hunger strike.
Mr Ratcliffe said prison officials have refused to let Nazanin, 40, see a doctor to examine lumps in her breast and other health issues.
He said his wife also needs neurological care over her neck pains and numbness in her arms and legs, as well as permission to see an outside psychiatrist.
He added that it was hoped the hunger strike would get the Iranians to take his wife "seriously".
Speaking at a news conference, Mr Ratcliffe set out his wife's demands - to be able to see an external doctor and to be given an agreement in writing that she will be allowed access to any medical treatment the doctor recommends.
'Enough is enough'
Mr Ratcliffe also described how his wife was told by members of Iran's Revolutionary Guard in late December that she would be "safer afterwards" if she spied on the UK Department for International Development and the civil society organisation Small Media.
He said this offer, which she refused, "really pushed her over the edge".
"The only weapon we have is to tell Nazanin's story," he added.
Speaking to the Observer, Mr Ratcliffe said: "We know a hunger strike has significant physical consequences the longer it goes on for, and Nazanin is feeling a strong sense of trepidation.
"But there aren't many ways she can say, 'enough is enough, take me seriously'."
Mrs Zaghari-Ratcliffe is going on hunger strike in Iran's Evin prison, alongside prominent Iranian human rights activist Narges Mohammadi, who also requires medical attention according to Amnesty International.
Mr Ratcliffe said his wife would consider extending the strike if her demands to see a doctor were not met.
Mrs Zaghari-Ratcliffe, of Hampstead in north-west London, spent her 40th birthday on Boxing Day in prison, where she has been detained for more than 1,000 days.
She was arrested at Tehran airport in April 2016 and has always maintained the visit was to introduce her daughter, Gabriella, to her relatives.
The four-year-old has been staying with family in Iran since her mother was detained - although they were briefly reunited during a three-day temporary release last August.
Since returning to prison, after her application for an extended release was denied, she has suffered several panic attacks.
Her husband has mounted a high-profile campaign for his wife's release, with Prime Minister Theresa May, Foreign Secretary Jeremy Hunt and London Mayor Sadiq Khan all calling for her to be freed.
Monique Villa, chief executive of the Thomson Reuters Foundation, where Mrs Zaghari-Ratcliffe works, said: "It is extremely shocking to see our colleague Nazanin Zaghari-Ratcliffe going on hunger strike to protest at her inhumane treatment in Evin prison, at a time where her health is already at its poorest.
"I am sincerely worried about the dire consequences of this.
"As her employer, I repeat that Nazanin is totally innocent, and certainly not spy material, as portrayed by the Iranian Revolutionary Guard.
"The Iranian government and judiciary must act immediately, release her and allow urgent treatment before her health deteriorates any further."