Iranian interrogators attempted to get a British-Iranian mother detained in Tehran to spy on the UK in exchange for her freedom, her husband has said.
Nazanin Zaghari-Ratcliffe was jailed for five years in 2016 after being convicted of spying, which she denies.
Her husband said Iran’s decision to use the tactic on her in late December meant they were in a “scary place”.
Mrs Zaghari-Ratcliffe, 40, has begun a three-day hunger strike in protest at being denied specialist medical care.
It comes as Foreign Secretary Jeremy Hunt summoned the Iranian ambassador to the Foreign Office to discuss the case.
Mr Hunt tweeted: “Her ongoing detention is totally unacceptable and her treatment at the hands of Iranian authorities is a fundamental breach of human rights.”
He said it was “a truly terrible indictment of Iran’s approach” that the aid worker felt the need to go on hunger strike, adding: “Iran must take action now.”
However, the Iranian ambassador criticised the UK’s “interference” in the case following his meeting with the foreign secretary, according to IRNA news agency.
Richard Ratcliffe, who met Mr Hunt on Monday afternoon, described how his wife was told by members of Iran’s Revolutionary Guard that she would be “safer afterwards” if she spied on the UK Department for International Development and London-based organisation Small Media.
He said this offer, which she refused, “really pushed her over the edge”.
Speaking at a news conference, Mr Ratcliffe said prison officials have refused to let his wife see a doctor to examine lumps in her breast and other health issues.
He set out her 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.
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.
Mrs Zaghari-Ratcliffe is going on an initial three-day hunger strike in Iran’s Evin prison, alongside prominent Iranian human rights activist Narges Mohammadi, but may strike for longer if her demands are not met.
Mr Ratcliffe added that it was hoped the hunger strike would get the Iranians to take his wife “seriously”.
After being summoned to the Foreign Office, the Iranian ambassador to the UK, Hamid Baeidinejad, told IRNA news agency: “The Iranian government considers any interference by the British government on this matter unacceptable.”
He was quoted as saying he had explained to Mr Hunt that Mrs Zaghari-Ratcliffe was considered an Iranian national and was entitled to her rights in prison – including access to medical facilities.
Mrs Zaghari-Ratcliffe, of Hampstead in north-west London, spent her 40th birthday on Boxing Day in prison, where she has been held for more than 1,000 days.
She was arrested at Tehran airport in April 2016 but 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.
Her husband has mounted a high-profile campaign for his wife’s release, with Prime Minister Theresa May, Mr 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 was “extremely shocking” to see Mrs Zaghari-Ratcliffe go on hunger strike.