Britain's efforts to improve relations with Iran are in danger of being wiped out by a diplomatic dispute over the choice of the next ambassador to Tehran.
The Iranian government has refused to accept the appointment of David Reddaway, an expert on the region who served a three-year stint as a chargé d'affaires in Iran during the early 1990s.
Iranian officials gave vent to their objections with the publication of an article in the Farsi-language daily newspaper Jomhuri Islami, claiming that the British diplomat is of Jewish origin and – without offering any supporting evidence – alleging that he is an MI6 agent.
A Foreign Office spokesman declined to comment on Mr Reddaway's religious background, pointing out that such appointments "are based on merit and not religion". Other sources said there was no basis for the MI6 claim. The Foreign Office is at pains to play down the affair. Officials refused even to confirm that Mr Reddaway, who is 48, had been nominated as ambassador, replacing Nick Brown who left the post last month.
"We are not commenting on the discussions going on with Iran," a spokesman said. "There is often a short gap between ambassadors."
However, the spat is clearly threatening to cast a new chill over fragile British-Iranian relations, which improved after the 11 September attacks when Britain sought to enrol support from Islamic nations for the US-led assault on Afghanistan.
A fortnight after the atrocities in America, Jack Straw became the first British Foreign Secretary to visit Tehran since the Islamic revolution in 1979. He made a second visit in November, when negotiations were being held over the new interim government to replace the ousted Taliban regime in Afghanistan.
The dispute has attracted the attention of Israel, which has made no secret of its disapproval of Britain's overtures to a nation that it sees as an arch enemy, and which will be watching the Foreign Office's handling of the affair closely.
The news comes as the government of the Israeli Prime Minister, Ariel Sharon, is trying to attract the world's attention to its seizure of an arms shipment from Iran which it claims was ordered by Yasser Arafat's Palestinian Authority.
Mr Reddaway is a high-flyer, who has spent 25 years in the Foreign Office.
His credentials for the post appear to be strong: he served in Tehran before the 1979 revolution, initially as an official in the embassy's commercial department and then as a political first secretary. He returned in 1990 as chargé d'affaires. He speaks Farsi, and has a wife of Iranian origin.
He also served in Madrid, Delhi and Buenos Aires, before being appointed head of the Foreign Office's Southern European Department. He is currently the Foreign Office's director of public services.