Foreign Secretary David Miliband has urged Iran to overturn a four year jail sentence reportedly given to a British embassy employee, BBC reported.
Hossein Rassam, 44, was arrested in June at the time of massive street protests over the country's disputed presidential election.
The Iranian, who worked as the embassy's chief political analyst, was accused of spying and inciting unrest.
In a statement, Mr Miliband called the reported sentence "wholly unjustified".
The foreign secretary said it represented "further harassment of embassy staff for going about their normal and legitimate duties".
He added the British ambassador in Tehran had spoken to Iran's deputy foreign minister and that the Iranian ambassador in London had been called in to explain the decision.
Mr Miliband said: "We understand the sentence can be appealed. I urge the authorities to conduct this quickly and overturn this harsh sentence."
He added: "We are in close touch with EU and other international partners, who continue to show solidarity in the face of this unacceptable Iranian action.
"This will be seen as an attack against the entire diplomatic community in Iran and important principles are at stake."
Nine British embassy employees were arrested but Rassam was the only one to be charged.
He was put on trial along with an employee of the French embassy and protesters angry at the results of the June elections.
Tehran accused the UK of fomenting opposition demonstrations when President Mahmoud Ahmadinejad was returned for a second term in office.
The British government has rejected the accusations.