Ghoncheh Ghavami, the British-Iranian woman sentenced to one year in prison in Iran for trying to attend a volleyball game, has gone on a “dry” hunger strike, meaning she is refusing food and water.

Her brother, Iman Ghavami, said she is protesting because the judge has yet to confirm her sentence.

“My family is quite devastated,” he said. “She is not eating any kind of food or drinking any liquids.”

Ghavami, 25, previously went on a hunger strike in early October to mark 100 days since she was imprisoned, but accepted fluids.

Iman Ghavami said his sister’s lawyer had seen the court document sentencing her to a year in jail, but a deadline to make the verdict formal had passed.

“The case is in limbo … I don’t understand why they don’t want to issue the verdict when they have made the decision,” he said. “It’s quite concerning for us. They have no legal basis to have her detained.”

Ghavami was arrested in June after trying to enter Tehran’s Azadi stadium along with male fans. Iran has a longstanding ban on women attending big sporting events with men.

She participated in a small protest in front of the stadium, wearing a white scarf and holding up a placard. Ghavami was initially released after a few hours, but was rearrested days later at a police station when she went to reclaim items that had been confiscated.

It emerged at the weekend that Ghavami has been convicted on a charge of “spreading propaganda against the ruling system” and sentenced to a one-year jail term. The Iranian judiciary has denied that her detention has anything to do with sport but has shed little, if any, light on why she remains in custody.

Amnesty International, which has described Ghavami as “a prisoner of conscience, arrested solely for taking part in a peaceful protest”, said on Tuesday: “It is deeply concerning that Ghoncheh Ghavami has found no other way to protest against the gross injustice of her predicament but by risking her health in embarking on another hunger strike.”

Faraz Sanei, of Human Rights Watch, said the case illustrated many of the flaws of Iran’s justice system. “The case against Ghavami exposes the spectacular failings of a revolutionary court system that regularly deprives the accused of a fair trial, brazenly flouts its own rules, and appears to punish detainees and their families by indulging in seemingly never-ending fishing expeditions to find ‘proof’ of guilt,” he said. “It’s well past time for the judiciary to put an end to this charade by immediately setting aside any judgments against Ghavami, dropping any and all outstanding charges, and allowing her to rejoin her family as a free woman.”

According to her brother, Ghavami was due to make a phone call to the family on Monday, which did not take place. “We are hoping to hear from her, to see how she is doing, especially now that she’s on a dry hunger strike,” he said.

More than 700,000 people have signed a petition calling on the Iranian authorities to release Ghavami.