Anti-British protests have been held outside the UK embassy in Tehran
Some UK embassy staff detained in Tehran and accused of inciting protests after disputed elections will face trial, a top Iranian cleric says.
Guardians Council chief Ahmad Jannati said: "Naturally they will be put on trial, they have made confessions."
Nine embassy staff were held last weekend. Britain says all but two have now been freed.
European Union governments summoned Iranian ambassadors to protest against the detentions.
An EU official told the BBC that, in addition, visas for Iranians holding Iranian diplomatic passports would be suspended.
The official said other measures, including the withdrawal of EU ambassadors from Iran, would be considered if the two staff members were not released.
Protests gripped Tehran and other Iranian cities after June's presidential election, amid claims the vote had been rigged in favour of the incumbent, President Mahmoud Ahmadinejad.
The Guardians Council - Iran's supreme legislative body, which Ayatollah Jannati heads - on Monday ratified his re-election, following a partial recount.
'Velvet revolution' plan
Ayatollah Jannati did not say how many employees would be tried or on what charges.
"In these incidents, their embassy had a presence, some people were arrested," he told the thousands of worshippers at Friday prayers, according to news agencies.
Ayatollah Jannati: "After the election, the enemy made an effort to poison the people"
Ayatollah Jannati said on Friday: "After the election, the enemy could not stand people's joy. The enemy made an effort to poison the people. They had planned a velvet revolution before the election."
He said the UK Foreign Office had warned of possible "street riots" around the 12 June election and had advised its nationals to avoid public places.
BBC diplomatic correspondent Jonathan Marcus says Ayatollah Jannati's speech marks a significant deterioration in the already bad relationship between London and Tehran.
Tehran has repeatedly accused foreign powers - especially Britain and the US - of stoking unrest after the election.
Britain has protested strongly against the arrests and rejected the Iranian allegations as baseless.
In the fallout from the crisis, Tehran expelled two British diplomats and the UK responded with a similar measure.
Foreign Secretary David Miliband said Britain was urgently seeking clarification from Iran about a possible trial and remained "deeply concerned" about the two staff members in detention.
"We are confident that our staff have not engaged in any improper or illegal behaviour," he said.
The Foreign Office later confirmed that Iranian envoy Rasoul Movahedian had been summoned and the same message reiterated.
Five of the nine employees were reportedly released on Monday and Iranian state media said on Wednesday it had freed three more, but British and EU officials say two remain in custody.
Iran's semi-official Fars news agency reported this week that one of the detainees had played a "remarkable role during the recent unrest in managing it behind the scenes".
Iran's Supreme Leader, Ayatollah Ali Khamenei, last month described Britain, as the "most evil" of its enemies.
Library ballot boxes
The issue of how to deal with Iran is set to dominate the summit of the Group of Eight (G8) industrialised nations in Italy next week.
Some EU countries have urged caution, arguing that Europe should engage with Iran, not isolate it.
But if the embassy staff are put on trial, the EU may have few other options than to tighten the diplomatic screw, correspondents say.
Meanwhile, the governor of one of Iran's biggest cities, Shiraz, has denied reports that a number of sealed ballot boxes in its main library contained votes from last month's election.
Ebrahim Azizi said the boxes were from previous polls and that the interior ministry had ordered they be archived there.
Earlier this week, an Iranian journalist posted pictures on the internet of several ballot boxes sitting on the floor of the library.