36 members of an Iranian opposition group were released in Iraq

Some of the British Embassy's Iranian staff held in Tehran will be put on trial, ruling clerics have said.

Ayatollah Ahmad Jannati made the announcement in a prayer sermon.

Downing Street said it was "concerned" about the reports.

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

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".

Earlier Iranian officials said all but one of the nine embassy personnel originally arrested had been released, but European Union officials said they believed more than one was still being held.

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

The trial announcement came a day after the European Union demanded Iran release the staff. Britain is pressing EU countries to pull their ambassadors out of Tehran in protest.