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consideration, and European Union officials had decided that the threat of such retaliation should not be completely withdrawn.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”.