The British-Iranian charity worker Nazanin Zaghari-Ratcliffe, being held in a jail in Iran, has undertaken a hunger strike over her treatment.
Her husband Richard Ratcliffe, who said he would join his wife in not eating, will begin a vigil in front of the Iranian embassy in London.
Ms Zaghari-Ratcliffe was arrested in April 2016 at a Tehran airport as she headed back to the UK with her daughter, Gabriella, after a family visit. She was sentenced to five years in jail on espionage charges, accusations she denies.
Her husband said Ms Zaghari-Ratcliffe had always planned to do "something" if she was still in jail when their daughter turned five years of age.
This is not her first hunger strike.
"Today I received a phone call from Nazanin in prison. She had informed the judiciary that she has begun a new hunger strike – she will drink water – to protest at her continuing unfair imprisonment," said Mr Ratcliffe on a petition page in support of her.
“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 said.
Mr Ratcliffe urged for British embassy officials to be allowed to check on his wife's health. He asked for Iranian authorities to grant him a visa if she was not released in the next few weeks.
UK foreign secretary Jeremy Hunt said on Twitter that Iran should “do the right thing” and “let this innocent woman” go home.
It comes only a day after Mr Hunt said that responsibility “almost certainly lies” with Iran over the recent attacks on oil tankers in the Gulf of Oman.
Iran summoned the British ambassador to Tehran on Saturday, the Students News Agency ISNA reported.
“During the meeting with Iran’s foreign ministry official, Iran strongly criticised Britain’s unacceptable stance regarding the attacks in the Gulf of Oman ... No other country but Britain has supported America’s accusations over the attacks,” the agency said.
The Sunday Times said the UK was sending 100 elite Royal Marines to a base in Bahrain to help with Naval patrols.