Ghoncheh Ghavami, 25, was arrested on June 20 outside the city's Azadi Stadium, where she and others were demanding that women be allowed in to watch a volleyball match between Iran and Italy.
She was charged by a Tehran court with activities and propaganda against the Islamic Republic. The semi-official ILNA news agency quoted her lawyer, Alireza Tabatabaie, on Saturday as saying her sentence may be cut due to previous good behaviour.
Britain, which has no permanent diplomatic presence in Iran but has said it plans to reopen its embassy soon, said it had several worries about the way Ghavami had been treated and now reportedly sentenced to a jail term.
"We are concerned about reports that Ghoncheh Ghavami has been sentenced to 12 months in prison for ‘propaganda against the state'," a spokesman from the Foreign Office in London said in a statement.
"We have concerns about the grounds for this prosecution, due process during the trial, and Miss Ghavami’s treatment whilst in custody.”
Iran does not recognise dual citizenship and treats dual nationals as Iranians.
Ghavami was released soon after her arrest in June but was re-arrested days later when she was called back to reclaim personal belongings that authorities had confiscated. In October, she went on a two-week hunger strike.
Iranian women in the Islamic Republic are banned from watching certain male sports events such as football and volleyball.
Tabatabaie said that, for "various reasons", he had been unable to meet his client in the lead-up to Saturday's hearing.
Ghavami's detention came shortly before the arrest in July of another dual-nationality citizen, Jason Rezaian, an Iranian-American reporter for the Washington Post, who is being held without charge.
His mother and brother called last week for the 38-year-old's release. "After 100 days it’s time for Iran to concede Jason’s innocence and release him," Mary Breme Rezaian and Ali Rezaian wrote in a statement posted online on Thursday.
The United States has no direct diplomatic relations with Iran.
(Reporting by Michelle Moghtader; Additional reporting by Andrew Osborn in London; Editing by Sami Aboudi and John Stonestreet)
