Ghoncheh Ghavami has stopped eating to mark 100 days since she was arrested in Tehran for trying to watch a volleyball game

Fears are growing over the health of a British-Iranian woman detained in Iran after she went on hunger strike to mark 100 days since she was imprisoned for trying to watch a volleyball game.

Ghoncheh Ghavami, 25, has not eaten for five days in protest at her detention in Tehran’s most notorious prison over a charge of “propaganda against the regime”.

In her first interview with western media, Ghavami’s mother, Susan Moshtaghian, 49, told the Guardian that she too had gone on hunger strike and was increasingly worried about her daughter’s condition.

“I am extremely concerned about Ghoncheh’s health and life beyond words. I hold authorities fully responsible for any harm inflicted on my daughter,” she said.

Ghavami – a law graduate of SOAS, University of London – was arrested on 20 June after trying to watch a men’s volleyball match at the Azadi stadium in Tehran, which put her in breach of the Islamic republic’s ban on women attending big sporting events. She spent nearly 50 days in solitary confinement in Evin prison and has been allowed only minimal contact with her family.

Moshtaghian, from London, was allowed to visit Ghavami for the first time in 19 days on Saturday. “She told me she has been on hunger strike since 1 October as objection to her uncertain conditions and 100 days of temporary custody with no basis,” she said.

“I am restless since I heard about this and I have also gone on hunger strike. I stayed silent for 82 days in the hope that my daughter comes back home safely. Now I am worried about her life and will not stop until she’s free.”

Ghavami’s brother, Iman, 28, said his sister had lost weight since she has been imprisoned and was visibly thinner when her parents visited. “People die from hunger strike. It’s a very serious situation and it’s been five days now,” he said.

Iman, who lives in London, is due to meet Foreign Office officials in London on Monday to discuss what can be done to help secure his sister’s release. A petition started by Iman on the site Change.org has so far amassed more than 527,000 signatures.

An FCO spokeswoman said: “We are concerned about the detention of Miss Ghoncheh Ghavami, a dual British-Iranian national in Iran. We are in touch with her family. We have raised our concerns with the Iranian government and asked for more information about her welfare and the charges against her.”

Ghavami was questioned for four hours after being arrested for trying to enter the stadium. She was released but rearrested days later when she tried to collect her belongings from a police station and officers discovered dual citizenship.

Ghavami was only told on 23 September that she was formally charged with “propaganda against the regime” – a charge that carries a possible prison term of several years. Her case is due to be heard by Tehran’s notorious revolutionary court.

Prison officials have declined to explain why they cancelled visits by Ghavami’s family for the past 19 days. It has also emerged that the family’s lawyer has been prevented from seeing Ghavami since she was detained.

Last week, another lawyer visited Ghavami in prison and pressed her to dismiss the family lawyer – raising concerns among her parents. “My parents are really upset at how he could gain access and meet my sister when the official lawyer hasn’t been able to,” Iman said. “How could someone just appear out of nowhere and force himself into the case?”

The Iranian president, Hassan Rouhani, was questioned about Ghavami’s detention, as well as that of a number of US-Iranian citizens, on Christiane Amanpour’s CNN show in September. He said: “We never wish for any individuals, Iranian or not Iranian, be it in Iran or in other countries, to be imprisoned or detained or put on trial.

“But naturally as a matter of course individuals can be suspected of something by the judiciary, by the security organisations and they are pursued. As the president it is my goal for the laws to be respected and I hope that regarding every single person you have named in your question … according to our laws they are Iranian citizens only, we do not accept dual-citizenship, but the bottom line is our aim is for the laws to be respected every step of the way.”