Richard Ratcliffe said he had received a phone call from his wife to tell him she had informed the Iranian judiciary she had stopped taking food in protest at her “unfair imprisonment”.
Ratcliffe said he would also not eat for the duration of her protest.
He said his wife’s decision followed the fifth birthday of their daughter, Gabriella, who has not been allowed to leave Iran following her mother’s arrest in 2016 and is living with her grandparents.
“This is something she had been threatening for a while,” Ratcliffe said.
“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’. This really has gone on too long.
Ratcliffe said his wife sounded “nervous, but calm” when she spoke to him from prison.
“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 he had previously been planning a “small event” outside the Iranian embassy in London to mark Gabriella’s birthday.
“Given Nazanin’s decision, I will also begin a continual vigil in front of the Iranian embassy, perhaps occasionally joined by friends and family,” he said.
“During this vigil I will also not eat, and will continue this fast until such time as her hunger strike ends.
“I vowed last time that if she ever went on hunger strike again, we would not leave her to go through this ordeal alone.”
Zaghari-Ratcliffe was arrested on 3 April, 2016, at Tehran’s Imam Khomeini Airport as she prepared to board a plane with Gabriella back to the UK after visiting relatives.
The 40-year-old is serving a five-year sentence in Tehran’s notorious Evin Prison.
Foreign Secretary Jeremy Hunt granted Zaghari-Ratcliffe diplomatic protection in March, but Tehran refuses to acknowledge her dual nationality.
In January, she refused food for three days in protest at the decision of the prison authorities to deny her access to medical treatment.
Ratcliffe 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.