A British-Iranian woman who was arrested in Iran after trying to attend a men's volleyball match has been sentenced to one year in jail, local media says, quoting her lawyer.

Ghoncheh Ghavami, a law graduate from London, was arrested in June at a Tehran stadium, where Iran's national volleyball team was to play Italy.

The 25-year-old went on trial last month.

Ghavami's lawyer said she has been found guilty of spreading anti-regime propaganda.

Her brother, Iman Ghavami, said his family was still waiting for a court hearing to officially announce the sentence.

"My parents are kind of shattered really. I mean they didn't expect this you know, she's already been through enough and now she's going to get a year," he said.

"They're running from one office to another to see if they can get some sort of leniency or they can make an appeal."

Earlier Ghavami's lawyer Alizadeh Tabatabaie was quoted in Iranian media as saying the judge had shown him the sentence, but no reason was given for the conviction.

Iranian officials have said Ghavami was detained for security reasons unrelated to the volleyball match.

Britain said on Sunday it was worried about the case and the way Ghavami had been treated.

"We are concerned about reports that Ghoncheh Ghavami has been sentenced to 12 months in prison for 'propaganda against the state'," the foreign office said in a statement.

"We have concerns about the grounds for this prosecution, due process during the trial and Ms Ghavami's treatment whilst in custody."

The "Free Ghoncheh Ghavami" Facebook page, where her friends and family campaigned for her release, features photographs of her set against the slogan: "Jailed for wanting to watch a volleyball match."

An update on the page on Sunday appeared to corroborate the one-year sentence but bemoaned the closed-door legal process that has prevailed in the case.

"This morning Ghoncheh's family and lawyer returned empty handed from branch 26 of revolutionary court," it said.

"It is not clear to her family and lawyer as to what the current legal basis of her detention is. A fair and just legal process according to Iran's legal framework is the basic right of every Iranian citizen. Why are these rights not upheld in Ghoncheh's case?"

Ghavami's arrest came after female fans and women journalists were told they would not be allowed to attend the volleyball match at Azadi stadium in the capital.

National police chief General Esmail Ahmadi Moghaddam said it was "not yet in the public interest" for men and women to attend such events together.

Women are also banned from attending football matches in Iran, with officials saying this is to protect them from lewd behaviour among male fans.

AFP