A British-Iranian woman jailed in Iran has been transferred to a hospital mental ward, her husband said.

Nazanin Zaghari-Ratcliffe, 40, was arrested at Tehran's Imam Khomeini airport while travelling with their young daughter in April 2016 and sentenced to five years in prison after being accused of spying, a charge she vehemently denies.

Her husband Richard Ratcliffe said she was transferred from Evin prison on Monday to the mental ward of Iman Khomeini hospital, in Tehran, where she is being held under the control of the Iranian Revolutionary Guard.

Her father said he visited the hospital on Tuesday but was not allowed to see his daughter, and that she has not been allowed to contact her family.

In a press release, the Free Nazanin Campaign said it is not known what treatment she is receiving or how long she is expected to remain in hospital.

The transfer comes after Mrs Zaghari-Ratcliffe went on hunger strike for 15 days last month in protest at her "unfair imprisonment".

Mr Ratcliffe also did not eat for the period in solidarity with his wife as he camped on the pavement outside the Iranian Embassy in London.

The couple's five-year-old daughter Gabriella has stayed in Iran with her grandparents since her since her mother's arrest.

READ MORE: Jailed British woman in Iran begins new hunger strike

Mr Ratcliffe said: "Nazanin hoped that her hunger strike would move the Iranian authorities, and it clearly has.

"Hopefully her transfer to hospital means that she is getting treatment and care, despite my distrust of just what pressures can happen behind closed doors. It is unnerving when we don't know what is going on."

"I am glad her dad has been down there to keep vigil outside.

"Mental hospital has its worries at the best of times - but particularly when kept isolated and under the control of the Revolutionary Guard."

"Even now it still seems like games of power and control are being played by the Iranian authorities - even at the point of hospitalisation.

"We hope again this is the beginning of the end. And yet, we were also here last summer.

"We will be following up with the new prime minister whenever that is decided to ensure he takes personal responsibility for Nazanin's case."

READ MORE: Boris Johnson raises concerns over jailed British mother during visit to Iran 

At the end of the dual hunger strike, Mr Ratcliffe said he believed the protest had been successful in raising the profile of his wife's case, with more than 100 MPs coming to show support to him in person, including Labour leader Jeremy Corbyn.

Earlier this year, Foreign Secretary and Tory leadership hopeful Jeremy Hunt granted Mrs Zaghari-Ratcliffe diplomatic protection in a bid to resolve her case.

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In a 2017 gaffe, Boris Johnson, his rival to become prime minister, said that Mrs Zaghari-Ratcliffe was in Iran "teaching people journalism" - despite her family's insistence she was there to visit relatives.

He has repeatedly said the responsibility for her continued detention lies with the Revolutionary Guard.