Nazanin Zaghari-Ratcliffe has been taken to hospital after suffering multiple panic attacks in prison in Iran.
The charity worker’s husband, Richard Ratcliffe, said his wife had collapsed at Evin prison in Tehran.
She fainted after bidding goodbye to her daughter and voluntarily returning to jail on Sunday following an emotional family reunion.
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Mr Ratcliffe told The Telegraph on Wednesday morning: "She had a panic attack and collapsed today, and had one yesterday also.
"So she was taken to the prison clinic earlier and we were told she might be hospitalised."
Mr Ratcliffe said she had compained on Tuesday of numb legs, painful headaches and rashes.
"I think it it feels like the physical symptoms of crushing disappointment since she went back into prison," he said.
"Certainly, yesterday the head of the prison was worried enough to come down and see her and ask her how she was and what’s going on. He clearly was very surprised that she’d been brought back into prison and asked her why and what’s happened."
Tulip Siddiq, MP for the Ratcliffe family’s Hampstead and Kilburn constituency, said: “So sad to hear that my constituent Nazanin has had another panic attack and is now in the prison hospital. How much more can this poor woman take? We need to bring her back home and reunite her with her daughter.”
Mrs Zaghari-Ratcliffe, a 40 -year-old British-Iranian, was released from prison on Thursday and was staying with family outside the capital during a three-day release.
Her husband earlier confirmed that her lawyer’s request for an extension was not granted.
Mr Ratcliffe said the prosecutors’ office told his wife she would need to return to the prison "for a few days or a week" to wait for a key signature.
She decided to go back voluntarily with her "head held high" rather than be "dragged out of the house in front of her baby", he said.
Nazanin Zaghari-Ratcliffe's detention
Mrs Zaghari-Ratcliffe’s four-year-old daughter, Gabriella, cried when she realised her mother was leaving.
He said: "She [Nazanin] promised Gabriella that the next time she saw her it would be forever not just for a few days, for proper freedom, not just for furlough.
"And next time they will go back to London to be with daddy."
Foreign Secretary Jeremy Hunt said he had spoken to Iran’s foreign minister, Mohammad Javad Zarif, on Friday but that "clearly wasn’t enough". Writing on Twitter, Mr Hunt vowed to continue the fight:
Looks like Iranian legal system is impervious to the simple fact at the heart of this: an innocent woman is desperate to be reunited with her family. Spoke to Foreign Minister Zarif on Fri but that clearly wasn’t enough. The fight goes on #FreeNazanin https://t.co/fknbIRC2lb
— Jeremy Hunt (@Jeremy_Hunt) August 26, 2018
Mrs Zaghari-Ratcliffe, from Hampstead, north London, was sentenced to five years in jail after being accused of spying by Tehran.
She denies the allegation and said she was on holiday in Iran to allow her daughter to spend time with relatives there.
Gabriella has been staying with family since Mrs Zaghari-Ratcliffe, who works for the Thomson Reuters Foundation, was detained at Imam Khomeini airport in April 2016.
Mr Ratcliffe said his wife was initially told she did not have to return to prison on Sunday and that her extension request had been authorised.
She was later told the permit had not been signed off and must return to prison "for a few days or a week" while this was sorted.
Mr Ratcliffe said the British embassy had been informed of the decision, but had reported hearing no mention of the charity worker needing to return for a few days to wait for a signature from the Iranian authorities.
Nazanin Zaghari-Ratcliffe timeline
Earlier on Sunday, Mrs Zaghari-Ratcliffe went to the prosecutors office with her daughter where she was warned not to do anything to jeopardise her chances of longer-term release.
He said: "She was shivering and shaking and crying – and said: ‘How can you take me away from my baby, when she needs me?’.
"Gabriella was crying and sucking her thumb – she didn’t want her mummy to go back."
Kate Allen, Amnesty International UK’s Director, called the Iranian authorities’ treatment of Mrs Zaghari-Ratcliffe "exceptionally cruel".
“Not only have they deprived her of her due process rights by locking her up for months in solitary confinement without access to a lawyer and subjecting her to a grossly unfair trial, but they’ve torn her away yet again from her daughter’s arms, subjecting them both to severe mental anguish and trauma," she said.
“Nazanin must immediately be given access to the specialised medical care she needs. Beyond that, she is a prisoner of conscience who has been unjustly jailed and must be released immediately and unconditionally."