British foreign secretary says he is 'urgently seeking' clarification about threat.
A powerful Iranian cleric said on Friday that British embassy local staff arrested for allegedly stoking unrest after the June election will be put on trial, a move that prompted coordinated protests from European governments.
Britain's foreign secretary David Miliband said he was "urgently seeking clarification" about the announcement as governments across the 27-nation European Union called in Iranian ambassadors.
"In these incidents, their embassy had a presence, some people were arrested. Naturally they will be put on trial, they have made confessions," Ahmad Jannati, head of Iran's Guardians Council, said at Friday prayers.
He did not say how many would go on trial or disclose the charges.
Nine local staff at the British embassy were arrested late last month. London has said that seven have been released while Iranian state television has said only one remains in custody.
Miliband said he was "deeply concerned" about staff being detained.
"We have noted the remarks by Ayatollah Jannati suggesting that some of our local staff in Iran may face trial," he said in a statement.
"We are urgently seeking clarification from the appropriate Iranian authorities. I intend to speak to Foreign Minister (Manouchehr) Mottaki.
"We are confident that our staff have not engaged in any improper or illegal behaviour. We remain deeply concerned about the two members of our staff who remain in detention in Iran."
Tehran accused the embassy employees of instigating riots in the unrest that followed the disputed re-election of President Mahmoud Ahmadinejad, which his rivals said was rife with fraud and irregularities.
Jannati, who is close to supreme leader Ayatollah Ali Khamenei and is a strong Ahmadinejad supporter, said "enemies" had been plotting a "velvet revolution" in the Islamic republic.
He said London had predicted "street riots" around the June 12 election and had warned Britons to stay away from public places.
Khamenei has described Britain, which has long had turbulent relations with Iran and a lengthy history of mistrust, as the "most evil" of its enemies.
EU governments called in Iranian envoys in protest.
Swedish state secretary for foreign affairs Frank Belfrage told Iranian ambassador Rasoul Eslami that Britain had the "full support" of the European Union. Stockholm currently holds the rotating EU presidency.
Belfrage said after the meeting that if Iran does not immediately free Britain's detained staff "the EU will take further action."
Swedish Prime Minister Fredrik Reinfeldt urged the EU to show "a united front" towards Iran while French President Nicolas Sarkozy expressed "total solidarity" with Britain.
"It's up to the British to tell us what they need," Sarkozy said at a joint news conference in Stockholm with Reinfeldt.
Iran lashed out at the West for "meddling" after an international outcry over the election and the repression of opposition protests.
At least 20 people were killed in street violence and many hundreds were rounded up in the most serious crisis since the 1979 revolution.
Britain appears to have replaced the United States, often dubbed "the Great Satan" by Iranian leaders, as Tehran's top foe in the wake of the election.
Last month, Mottaki said Iran may downgrade ties with Britain, after the two governments expelled each other's diplomats. Tehran has also expelled the BBC correspondent and arrested a British-Greek reporter.
Iranian officials have been particularly angered by this year's launch of the BBC's Persian satellite channel, which they accuse of fanning the flames in the election dispute.
The roots of mutual distrust date back to the 1800s when Iran, then Persia, was trapped in the colonial rivalry between Russia and Britain.
In 1953, nationalist prime minister Mohammad Mossadegh was overthrown in a CIA-organised coup with support from British operatives after he nationalised the Anglo-Iranian oil company, the forerunner to British Petroleum.
Diplomatic ties were severed when Britain's mission in Tehran was closed in 1980 after British special forces stormed the Iranian embassy in London to end a hostage siege.
A 1989 fatwa by Irans revolutionary leader Ayatollah Ruhollah Khomeini against British writer Salman Rushdie sparked a new rupture in ties that were only restored in 1999.
In 2007, Iran seized 15 British navy personnel on patrol in waters between Iraq and Iran and held them for 12 days.