A powerful Iranian cleric suggested on Friday that detained local staff from the British embassy in Tehran would be put on trial in the Islamic Republic for their alleged role in post-election unrest.

In London, Britain's Foreign Office said it was "very concerned" about Ayatollah Ahmad Jannati's statement that local embassy staff may face trial in connection with demonstrations against last month's disputed presidential election.

Iran earlier this week said nine Iranian British embassy staff had been detained for involvement in mass street protests that erupted after the election won by hardline President Mahmoud Ahmadinejad.

Most have since been released, but British officials say two embassy employees remain in detention.

"In these developments their embassy here maintained a presence (in the unrest) in which individuals were arrested and inevitably they will be tried as they have (made) confessions," Jannati told Friday prayer worshippers in Tehran.

Jannati is a conservative who heads the Guardian Council, a powerful 12-member constitutional watchdog.

Britain has denied Iranian accusations that embassy staff were involved in instigating opposition protests after the vote, which defeated presidential candidate Mirhossein Mousavi says was rigged in the incumbent's favour.

"VELVET REVOLUTION"

The European Union has pledged a strong, collective response to any Iranian harassment of staff at European embassies.

Britain and Iran have already expelled two of each other's diplomats since the election, which stirred Iran's most striking display of internal dissent since the 1979 Islamic revolution and strained ties with the West.

In London, a Foreign Office spokeswoman said: "We are very concerned by these reports and are investigating."

The spokeswoman added: "Allegations that our staff are involved in fomenting unrest are wholly without foundation. We will be seeking an urgent explanation from the Iranians."

She said British authorities were speaking to the Iranian embassy in London and to Iranian officials in Tehran.

On Wednesday, Iran's semi-official Fars News Agency said that one of those detained "had a remarkable role during the recent unrest in managing it behind the scenes".

It said another embassy employee had been a "main element behind the riots" but had been freed because she had diplomatic immunity.

Iranian officials deny the election was rigged, saying it was the nation's "healthiest" since the 1979 Islamic revolution.

The authorities have portrayed the unrest as the work of local subversives and foreign powers, especially Britain.

Jannati reiterated accusations by other senior Iranian figures that the West had planned a so-called "velvet" revolution to undermine the Islamic Republic's establishment.

"They (the British) had ahead of time ... announced that in the election that is scheduled to take place in Iran there might be unrest and turmoil," Jannati said.