Downing Street said last night it was “concerned” about reports that British Embassy staff in Tehran will face trial.

Prime Minister Gordon Brown's spokesman said urgent clarification was being sought from the Iranian government regarding the claims.

Top Iranian cleric Ayatollah Ahmad Jannati made the announcement in a prayer sermon earlier yesterday.

Eight embassy staff were arrested at the weekend amid rapidly deteriorating relations with Iran since the disputed re-election of president Mahmoud Ahmadinejad.

Mr Brown's spokesman said: “We are aware of these reports. We are concerned about these reports and we are seeking clarification on this as a matter of urgency.”

Jannati, head of Iran's powerful Guardian Council, did not say how many of the embassy's Iranian staff, held since the weekend, would be tried or on what charges.

But he claimed that the staff “made confessions”.

Iranian state media previously claimed that all but one of those arrested had been released.

They were accused by the Iranian authorities of being involved in anti-government protests.

A Foreign Office spokesman said: “We are very concerned by these reports and are investigating. Allegations that our staff were involved in fomenting unrest are wholly without foundation.

“We will be seeking an urgent explanation from the Iranians.”

There have been frequently conflicting claims about how many embassy staff were initially detained and how many have subsequently been released.

The Foreign Office said nine were arrested on Saturday. Downing Street said on Monday that, of those, four remained in custody.

On Wednesday, the Foreign Office said two more had been released. Iranian officials and state media said eight were arrested and have made varying assertions about how many were freed.

State broadcaster Press TV claimed on Wednesday that all but one had been released.

But Jannati's comments yesterday suggested that at least two were still being held.

“In these events, their embassy had a presence,” he said. “Some people were arrested. Well, inevitably, they will be put on trial.”

The weekend arrests intensified the diplomatic row which has escalated between the UK and Iran in recent weeks.

Britain has criticised Iran's crackdown on protesters while the Tehran regime has accused the UK of inciting unrest.