Charity worker Nazanin Zaghari-Ratcliffe is going on hunger strike to protest against her "unfair imprisonment" inside an Iranian prison.
Her husband Richard said he received a phone call from his wife that she would stop eating to draw attention to her case and put pressure on the British and Iranian government.
Mr Ratcliffe will be carrying out his own hunger strike outside the Iranian embassy in London throughout the duration of her protest.
The British-Iranian mother was sentenced to five years in jail after being accused of spying by Tehran’s Islamist regime, a charge she vehemently denies.
Her hunger strike marks her daughter Gabriella's fifth birthday, in a demonstration to both the British and Iranian government that action must be taken.
Gabriella has not been allowed to leave Iran following her mother’s arrest in 2016 and is living with her grandparents.
In a statement this morning, Mr Ratcliffe said: "Today she sounded nervous, but calm.
"Her demand from the strike, she said, is for unconditional release. She has long been eligible for it."
He added: "Given Nazanin’s decision, later today I will also begin a continual vigil in front of the Iranian Embassy, perhaps occasionally joined by friends and family.
"During this vigil I will also not eat, and will continue this fast until such time as her hunger strike ends."
Amnesty International UK described the situation as "heartbreaking".
Kate Allen of Amnesty International UK said: “Nazanin has already been through so much, while her tireless husband Richard has strained every sinew to get Nazanin out of jail and back to the UK where she belongs.
“Nazanin is a prisoner of conscience, unfairly jailed after a sham trial and subjected to all manner of torments - including months in solitary conferment and endless game-playing over whether she would receive vital medical care.
“It’s shocking that it’s come to this, and we and countless people across the county fervently hope the Iranian authorities will now finally do the right thing and release Nazanin.
In March, Foreign Secretary Jeremy Hunt granted Mrs Zaghari-Ratcliffe diplomatic immunity.
However Iran does not recognise dual-nationality and said the UK's actions were illegal.