A senior Iranian employee of the British Embassy in Tehran has been given a four-year prison sentence after being found guilty of fomenting violence at the behest of the British Government, The Times has learnt.

Hossein Rassam, 44, the embassy’s political counsellor, was sentenced in a closed courtroom this week, although the outcome is yet to be publicly announced. The Foreign and Commonwealth Office learnt of his sentence on Tuesday and summoned the Iranian ambassador in protest. The British ambassador in Tehran has also lodged an official complaint.

Mr Rassam was one of eight Iranian staff at the British Embassy arrested after mass street protests that erupted in cities across Iran following the disputed re-election of President Ahmadinejad on June 12.

Iran’s Supreme Leader, Ayatollah Ali Khamenei, blamed the protests by opposition supporters on a British plot to bring down the regime. Britain denies any involvement. The embassy staff were among hundreds of people rounded up and detained after the disturbances. Seven others were released without charge but Mr Rassam was sent to the notorious Evin prison in Tehran and charged with being the “kingpin” behind a British plot.

In a statement to The Times, David Miliband, the Foreign Secretary, said that Mr Rassam’s sentence was unacceptable and urged that it be immediately repealed. He dismissed the charges against Mr Rassam as “wholly without foundation”.

He added: “We understand the sentence can be appealed. I urge the authorities to conduct this quickly and overturn this harsh sentence. Such a decision is wholly unjustified and represents further harassment of embassy staff for going about their normal and legitimate duties.”

Mr Rassam is still on bail after his release from Evin prison in August. It is unclear whether he will have to return to jail immediately or remain on bail pending his appeal. Last night he was returning from a trip to the north of Tehran to break the news of his sentence to his elderly mother. His wife and son are no longer in the country.

Foreign journalists were barred from attending any of Mr Rassam’s hearings but the state news agency reported that he had told the court that a £300,000 budget had been allocated to establishing contacts with political groups before the election, including Mir Hossein Mousavi, the reformist opposition candidate who claims that he was robbed of victory.

Mr Miliband warned of negative consequences for Iran from countries other than Britain, calling Mr Rassam’s treatment “an attack on the entire diplomatic community”.

He was arrested on June 27 and accused of “acting against national security” — a catch-all charge for any kind of political dissent. He has worked at the embassy since 2004.

The first details of other charges were published in July by Fars, the state news agency, which also reported that he had given confessions that would “cast light on many hidden angles of the interference of Britain in Iran’s internal affairs in recent years”.

Fars said that Mr Rassam was accused of spying for Britain and feeding anti-Iranian reports and “internal intelligence” to the British Government in his role as the embassy’s chief political analyst. A government newspaper headline that greeted his first court appearance in August read: “The British Embassy: headquarters for the coup command.”

Fars reported: “He has proved his strong anti-Iranian approaches by linking British ambassadors with elements from anti-government spectrums.” Mr Rassam had also provided “internal intelligence” to Sir John Sawers when he served as political director of the Foreign Office, it said. Sir John is now head of M16, where one of his most important jobs is to oversee intelligence gathering on what Britain suspects is Iran’s nuclear weapons programme.

Mr Rassam was also accused of giving “strategic advice” to foreign journalists in Iran including the BBC, whose correspondent was expelled after allegations of provoking unrest. Fars said that Mr Rassam had been arrested for attending a demonstration on June 28. In fact, the embassy employees were arrested a day earlier.

News of Mr Rassam’s sentence came as Britain and other major powers awaited Iran’s reply on a proposed deal over its uranium stockpiles that may determine the future of diplomatic negotiations with the country.

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