Laleh Shahravesh, 55, faces two years in jail and a £50,000 fine over posts allegedly written three years ago after she discovered that her former partner, Pedro, had remarried, the Detained In Dubai campaign group said.
Ms Shahravesh, from Richmond, south-west London, was arrested with her daughter Paris, 14, at Dubai Airport when the pair visited on 10 March for Pedro’s funeral, a week after his death from a heart attack, aged 51.
They were held for 12 hours before Ms Shahravesh’s passport was seized, Detained In Dubai said.
The teenager, who has since returned to Britain, has written to the prime minister of the United Arab Emirates and ruler of Dubai, Sheikh Mohammed bin Rashid Al Maktoum.
She said in the letter, published by Detained in Dubai: “I have not seen my mother in 23 days, and with every passing day, I feel less hopeful of her return.
“I ask kindly: please, please return my mother’s passport, and let her come home.”
Ms Shahravesh was married to Pedro for 18 years and they lived in Dubai, where he worked for HSBC, for eight months before she returned to Britain with their daughter. Their relationship broke down. A few months later, in 2016, she unexpectedly received divorce papers and saw from photos on Facebook that Pedro, who was Portuguese, had remarried.
In one post, she wrote: “I hope you go under the ground you idiot. Damn you. You left me for this horse.”
In another, which she wrote in Britain, she said: “You married a horse you idiot.”
Ms Shahravesh’s posts, written in Farsi, were reported by Pedro’s new wife, Samah Al Hammadi, 42, from Tunisia, Detained In Dubai said.
Ms Shahravesh said: “I had no idea he was getting married again, and so soon after our own marriage broke down.
“I reacted badly. I lashed out and wrote two unpleasant comments about his new wife on his Facebook page.”
The group said her arrest, under strict cybercrime laws which also include a ban on sharing charity pages online, was “simply unreasonable”.
In her letter to Sheikh Mohammed, Paris said of the initial detainment: “I cannot emphasise enough how scared I felt, especially after losing my father just a week before, as I was having to worry about losing my mother as well.
“Yet even though I felt terrified on the day that we arrived, the sick feeling in my stomach only became worse.”
She said her crying mother had been yelled at by police and had to sign a document in Arabic which she did not understand, and that the arrest had left her “without the closure that I had wished to gain from attending my father’s funeral”.
Jeremy Hunt, the Foreign Secretary, told reporters in Luxembourg the Government was “concerned” by the situation.
“Our diplomats in the UAE have enormous experience in dealing with consular cases as we saw from the Matthew Hedges case and so she is getting the best possible service from the FCO,” he said.
Mr Hedges was pardoned by the UAE in 2018 after he had been given a life sentence for spying for MI6, which was denied by Britain.
Ms Shahravesh must remain in the country and faces further court proceedings on Thursday.
Ms Shahravesh added: “I am terrified. I can’t sleep or eat. I have gone down two dress sizes because of the stress.
“And my daughter cries herself to sleep every night.
“We are so close, especially since her father left us and we only have each other. It breaks my heart to be kept apart from her.”
She said she has lost her job at a homeless shelter, could lose the flat she shares with her daughter and has borrowed £5,000 from her family.
“My life is in ruins, and that is even before the huge fines and jail I am facing here,” she said.
Radha Stirling, chief executive of Detained In Dubai who represents Ms Shahravesh, said the country’s cybercrime laws render “almost every visitor to the country a criminal”.
“I have spoken with Laleh, her mother, sisters and daughter Paris,” she said.
“Their experience is heartbreaking. Not only has Paris lost her father, but in going to visit him to say her final goodbye, she wound up in a frightening Middle Eastern police station, and is now without her mother.”
Responding to Mr Hunt’s comments, she said: “Laleh and her family are unaware of any diplomatic intervention and feel wholly abandoned by their government.”
A Foreign Office spokesman said its staff are supporting a British woman and her family following her detention in the United Arab Emirates.
The spokesman added: “We are in contact with the UAE authorities regarding her case.”