Foreign Secretary David Miliband said Tuesday's closed-courtroom sentence against Hossein Rassam, 44, an Iranian national working as the chief political analyst at the British Embassy, should be overturned on appeal.
"I urge the authorities to conduct (an appeal) quickly and overturn this harsh sentence," Miliband said in a statement.
"Such a decision is wholly unjustified and represents further harassment of embassy staff for going about their normal and legitimate duties."
Rassam, who has worked for the embassy since 2004, was sentenced to four years in jail and banned from working for any foreign embassies for at least five years after being found guilty of espionage for allegedly fomenting violence during the unprecedented post-election unrest that followed the disputed June 12 re-election of President Mahmoud Ahmadinejad, The Times of London reported Thursday.
Britain is "in close touch" with European Union and other international partners "who continue to show solidarity in the face of this unacceptable Iranian action," Miliband said.
He said the sentence was "seen as an attack against the entire diplomatic community."
Rassam, free on $200,000 bail, has until Nov. 16 to appeal the verdict, The Financial Times reported.
Tehran arrested Rassam June 27, accusing him of "acting against national security." It also arrested eight other local British Embassy workers following the election, accusing them, too, of stirring up unrest against the government.
The unrest protesting the election results included hundreds of thousands of people demonstrating in the streets, with some clashing with police, breaking into shops, tearing down signs and smashing windows.