A threat by Iran to put British Embassy staff on trial over alleged involvement in anti-government protests further heightened diplomatic tensions today.
Foreign Secretary David Miliband said he was seeking urgent talks with his counterpart in Tehran after a leading cleric in Iran announced the intended court action in a prayer sermon.
Ayatollah Ahmad Jannati, head of Iran's powerful Guardian Council, claimed staff had made confessions but did not say how many faced trial or on what charges.
"In these events, their embassy had a presence," he said. "Some people were arrested. Well, inevitably, they will be put on trial," he said.
Mr Miliband said: "We have noted the remarks by Ayatollah Jannati suggesting that some of our local staff in Iran may face trial.
"We are urgently seeking clarification from the appropriate Iranian authorities. I intend to speak to Foreign Minister 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."
Embassy staff were arrested at the weekend amid rapidly deteriorating relations with Iran since the disputed re-election of president Mahmoud Ahmadinejad.
Shadow foreign secretary William Hague condemned the prospect of a "show trial" and demanded a strong reponse from other European Union countries.
"The prospect of a show trial of any British Embassy staff in Tehran is utterly unacceptable. We call again for their immediate release.
"The harassment of the staff of any Embassy is a violation of diplomatic norms and conventions. This is a matter of principle and we should be able to expect solidarity and a united response from other European countries if Iran continues down this unwise path.
"A weak response from Europe would encourage Iran to think that it can try to bully and divide the international community with impunity."
Britain wants European Union nations to recall their ambassadors from Iran in protest at the detentions but no decision has yet been taken on such a joint action.
Carl Bildt, the foreign minister of Sweden which holds the rotating EU presidency, will chair forthcoming talks with fellow EU foreign ministers on possible sanctions against the regime in Iran, including a possible visa ban on travel to the EU by senior regime officials.
Their next scheduled meeting is not until July 27, but Mr Bildt is expected to call an earlier meeting if necessary to activate what the EU has promised will be a "strong and collective" response to any "harassment or intimidation" of embassy staff.
"It is not acceptable to file charges against the ones released or to the ones still in custody," he said.
French President Nicolas Sarkozy expressed "total" solidarity with Britain in attempts to secure the release of the embassy staff and urged stronger sanction against Iran "so that Iranian leaders will really understand that the path that they have chosen will be a dead end".
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 and curbing of freedoms while the Tehran regime has accused the UK of inciting unrest on the capital's streets.
It has already expelled two British diplomats from Iran - an action met with tit-for-tat expulsions by the UK.
There have been frequently conflicting reports 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 today suggested that at least two were still being held for being involved in the recent demonstrations in Iran.
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