London(CNN) Nazanin Zaghari-Ratcliffe's husband has begun a hunger strike outside the Iranian Embassy in London, in solidarity with his wife who started her third prison hunger strike in Iran on Saturday.

The British-Iranian charity worker took the decision to go on hunger strike again Saturday after her daughter Gabriella turned five years old on June 11 while she remains locked up, her husband Richard Ratcliffe told CNN from outside the embassy on Sunday.

"She's not playing now," he said. Zaghari-Ratcliffe has been detained for more than three years in Iran on spying charges.

Ratcliffe also began his hunger strike Saturday and has pitched a tent outside the embassy to protest his wife's continued detention. "Hunger strikes in prison can be ignored whereas on your doorstep is a different thing," he said.

Nazanin took the decision to go on hunger strike after her daughter Gabriella turned five while she remains incarcerated in Iran.

"I've just spoken to her, she was saying she was feeling her body is beginning to slow down," Ratcliffe said. He explained he is also starting to feel the physical effects of no food but is drinking water and mint tea.

Zaghari-Ratcliffe was allowed to see her daughter Sunday and told her about the hunger strike. "She said, 'does that mean mummy and daddy are going to die?'," Ratcliffe said, adding that his daughter is beginning to understand more about her mother's situation. He was hoping to speak to his daughter as the UK celebrates Father's Day on Sunday.

Ratcliffe said his wife was visited by representatives of the Iranian Prosecutor's Office on Sunday and they warned her that going on a hunger strike would not be good for her case.

Asked how long she will strike for, Ratcliffe said he presumes more than six days, which is her longest hunger strike so far. "We will take one day at a time," he said, confirming that he intends to strike for as long as she does.

A vigil was held for Nazanin outside the Foreign Office in London in 2018.

He decided to join his wife after she told him Saturday that she had submitted a letter to the Prosecutor's Office informing them of her hunger strike. "She sounded calm but said she was feeling a bit nervous," he explained.

"It's a really simple way of giving a message and not much else seems to work," he added.

Iran's ambassador to the UK, Hamid Baeidinejad, reacted to Richard Ratcliffe's protest Saturday, claiming in a tweet that demonstrators were blocking the entrance to the embassy. Ratcliffe refutes that they are blocking the entrance.

Amnesty International's UK director, Kate Allen, said in a statement: "This is a truly heartbreaking situation. 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 currently serving a five-year prison sentence after being convicted of "membership of an illegal group."

"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," she added.

Zaghari-Ratcliffe was arrested at Tehran's Imam Khomeini Airport on April 3, 2016, prior to boarding a plane back to the UK after a regular family visit to the country with her infant daughter.

She was sentenced to five years in prison in September 2016 after being convicted of "membership of an illegal group" by a Revolutionary Court in Tehran. She is presently serving her jail sentence in Tehran's notorious Evin Prison.

Asked whether he is anxious about an upcoming change in leadership in the UK and Boris Johnson potentially becoming Prime Minister, Ratcliffe said: "We never got to meet Theresa May as Prime Minister... so knowing Boris has its advantages."

In his most serious misstep as foreign secretary, Johnson was forced to apologize in 2017 after his comments about Zaghari-Ratcliffe raised fears that her jail sentence could be prolonged.

Johnson told a parliamentary committee that Zaghari-Ratcliffe had been teaching journalism during her visit to Iran, but he later clarified that she had been visiting relatives before she was detained.