Ghoncheh Ghavami, 25, has been sentenced to a year in jail after trying to attend a men’s volleyball game in the Iranian capital of Tehran.
Ms Ghavami, a graduate of the University of London School of African and Oriental Studies, has been detained since 20 June when she tried to attend a men’s volleyball match between Iran and Italy.
Iran banned women from volleyball games in 2012, extending a long-standing ban on football matches.
Ms Ghavami was found guilty of “propagating against the ruling system” during a secret hearing at Tehran’s Revolutionary Court, her lawyer said.
Mahmoud Alizadeh Tabatabaei said he had been shown the text of the court’s verdict but is still waiting to officially receive it.
Ms Ghavami, from Shepherd’s Bush in west London, was taking part in a protest against a ban on women in Iran attending sporting events in the company of men in public stadiums.
Amnesty International’s UK director Kate Allen described the verdict as “appalling” and called for Ghavami’s immediate release.
“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,” said Allen.
“Ghoncheh is a prisoner of conscience and the Iranian authorities should quash the sentence and release her immediately and unconditionally.”
More than 700,000 people have signed a petition urging the Iranian authorities to release Ghavami.
Last month, Miss Ghavami went on “wet” hunger strike – refusing all food but taking liquid – for 14 days, in protest at the conditions of her detention. She ended the hunger strike on 14 October.
A Foreign and Commonwealth Office spokeswoman said: “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.”