Nazanin Zaghari-Ratcliffe, the British-Iranian woman sentenced to five years in an Iranian prions, has been released from jail for three days on temporary leave.
Pictures posted online showed Mrs Zaghari-Ratcliffe, who was imprisoned in 2016, celebrating with her daughter Gabriella.
Foreign Secretary Jeremy Hunt vowed the Foreign Office would continue to press for her permanent release.
The Free Nazanin campaign said on Thursday, “Nazanin was released from Evin prison on furlough this morning. Initially the release is for three days - her lawyer is hopeful this can be extended.”
Many of the 39-year-old's cellmates have been released initially for three days "on furlough" - meaning temporary leave - but have been extended to over a month.
In a press release Mrs Zaghari-Ratcliffe said, “It will be just awesome for Gabriella to have mummy home finally.
“The thought of brushing her hair and giving her a bath. of being able to take her to the park, and feed her, and sleep next to her – it just kills me. It is still so hard to believe.”
“I was so emotional to see my grandmother today. I cried so much. I felt so overwhelmed. My dad’s home is not my home – but it is so much better than prison.”
“People in the ward were so excited – they sang songs and danced. I baked for them in celebration. It felt like this really could be the beginning of the end.”
Mrs Zaghari-Ratcliffe was given 10 minutes notice on Thursday morning that she was being released.
She travelled to Damavand from Evin with her father and brother to see wider family including her daughter Gabriella.
Foreign Secretary Hunt tweeted that it was "really good news" to hear she had been given temporary relief, adding that it was a "gross injustice" that she was in prison at all.
Richard Ratcliffe, Mrs Zaghari-Ratcliffe's husband, said news of her temporary release was "a genuine surprise after all the disappointments".
“I want to thank Jeremy Hunt for all his efforts, and for the efforts of all of his staff and all the Iranian authorities involved. I have been very critical in the past – I may need to be critical again", he said.
"But today is a good day, hopefully the turn of a new leaf as well as stone. It happened thanks to the personal care of the many people involved in Tehran, London and around the world.”
Mrs Zaghari-Ratcliffe was first arrested in Iran as she attempted to leave the country in 2016.
She was imprisoned for five years on charges of spreading propaganda, but her family maintain she was in the country on a holiday.
In November 2017, her case achieved new prominence after former foreign secretary Boris Johnson appeared to tell a parliamentary committee that Mrs Zaghari-Ratcliffe was in Iran training journalists - something the Iranian authorities accused her of, but which she denied.
Mr Johnson later acknowledged he had misspoken.
It was further compounded by remarks made by Environment Secretary Michael Gove, who said he did not know why she was in Iran.