Ghoncheh Ghavami, a British-Iranian woman from Shepherd's Bush in London, has reportedly been sentenced to a year in prison for trying to watch a men's volleyball match.
Her lawyer, Alizadeh Tabatabaie, told the BBC she'd been found guilty of spreading anti-regime propaganda.
The Iranian authorities banned women from volleyball games two years ago, arguing that women need protection from the behaviour of male fans.
The 25-year-old was arrested on June 30 outside Tehran's Azadi Stadium for taking part in a peaceful protest demanding women be allowed inside the venue.
In a statement, Amnesty International UK Director Kate Allen said:
This is an appalling verdict.
It's an outrage that a young woman is being locked up simply for peacefully having her say about how women are discriminated against in Iran.
Ghoncheh is a prisoner of conscience and the Iranian authorities should quash the sentence and release her immediately and unconditionally.
The authorities should also investigate allegations that Ghoncheh was subjected to death threats by her interrogators and provide compensation for her arbitrary detention and her prolonged solitary confinement.
Last month, after more than 100 days inside Evin prison in Tehran, Ghavami began a "wet" hunger strike — refusing to take food, but accepting liquid.
In a statement, the Foreign Office said it was "concerned" by the reports:
We are concerned about reports that Ghoncheh Ghavami has been sentenced to 12 months in prison for 'propaganda against the state'. We have concerns about the grounds for this prosecution, due process during the trial and Miss Ghavami's treatment whilst in custody.