In London, the Foreign Office said it was urgently checking reports that the Iranian authorities planned to put two of its local employees on trial.

Nine staff members were seized after the unrest sparked by Iran’s disputed presidential elections on June 12, and as many as eight of them were subsequently reported to have been released. But the precise number still detained was not clear.

Iranian state television said all but one of the nine had been released. But Foreign Minister Carl Bildt of Sweden, which currently holds the EU’s presidency, said “more than one” staff member remained in custody.

The Iranian authorities accused the employees of fomenting and orchestrating protests, but pro-democracy Iranians ascribed the violence on the streets to a widespread crackdown by Government security forces.

A spokeswoman for the Foreign Office in London said: “We are very concerned by these reports and are investigating. Allegations that our staff is involved in fomenting unrest are wholly without foundation. We will be seeking an urgent explanation from the Iranians.”

Britain has been pressing EU to withdraw all its ambassadors from Tehran. European officials have indicated that any talk of a trial - or show trial - would hasten those measures.

News reports on Friday quoted Ayatollah Ahmad Jannati, the head of the influential Guardian Council and an ally of Iran’s supreme leader Ayatollah Ali Khamenei, as telling worshippers during Friday Prayers in Tehran that the local employees would be tried after they “made confessions”.

Ayatollah Jannati did not say how many of the British detainees would be tried or what charges they would face, news reports said.

Reuters quoted him as saying: “In these developments their embassy here maintained a presence in which individuals were arrested and inevitably they will be tried as they have made confessions.”

Until Thursday, European Union diplomats meeting in Stockholm were searching for ways to resolve the standoff without withdrawing ambassadors from Tehran. But Swedish officials said an Iranian move to put the British Embassy personnel members on trial would escalate the dispute and strengthen Britain’s demand for recalling the ambassadors.

Before the Iranian threat on Friday, EU diplomats said, withdrawal of ambassadors was one of several options under consideration, and European Union officials had decided that the threat of such retaliation should not be completely withdrawn. Once the threat was made, however, the situation shifted, the diplomats said.

Hassan Qashqavi, Iranian Foreign Ministry spokesman, said on Monday that Iran was keen to maintain diplomatic relations with EU, its biggest trading partner. But on Wednesday, the semi-official Fars news agency said one of the employees, who was not identified by name, “had a remarkable role during the recent unrest in managing it behind the scenes”.