The chief political analyst at the British Embassy in Iran has been charged with "acting against national security", reports suggest.
The UK Foreign Office is investigating claims by his lawyer that he has been charged and will stand trial shortly.
A senior cleric has said some of the nine embassy staff arrested last month will be tried for inciting protests over Iran's disputed election.
Britain denies fomenting discontent to undermine Iran's Islamic regime.
Iranian news agencies have said all but one of the embassy staff have been released, although the UK government claims two remain in custody.
A Foreign Office spokeswoman could not confirm reports that the adviser had been charged.
'Deeply concerned'
"We are still investigating. The situation remains extremely unclear and fluid," she added.
Foreign Secretary David Miliband has said he is "deeply concerned" about the situation.
The BBC's world affairs correspondent Caroline Hawley said Mr Miliband had been unable to speak to his Iranian counterpart about the situation.
Our correspondent also said the embassy worker's lawyer Abdolsamad Khorramshahi had been unable to meet his client in Evin prison in Tehran where he is being held, or see the text of the indictment.
"This is a move certain to further fuel tensions between Britain and Iran," she said.
"Iran appears determined to blame outsiders for the worst political crisis the Islamic regime has ever faced," she added.
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.
On Friday, Ahmad Jannati, leader of the Iran's supreme legislative body the Guardian Council, said: "The British Embassy had a presence and some people were arrested.
"Well inevitably they will be put on trial. They have made confessions too."
However, he did not say how many employees would be tried or on what charges.
'Remarkable role'
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".
Mr Milband said: "We are confident that our staff have not engaged in any improper or illegal behaviour."
The Foreign Office later confirmed that Iranian envoy Rasoul Movahedian had been summoned and the same message reiterated.
European Union governments have summoned Iranian ambassadors to protest against the detentions.
IRAN UNRESTAn 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.
Tehran has repeatedly accused foreign powers - especially Britain and the US - of stoking unrest after the election.
In the fallout from the crisis, Tehran expelled two British diplomats and the UK responded with a similar measure.
Iran's Supreme Leader, Ayatollah Ali Khamenei, last month described Britain, as the "most evil" of its enemies.
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.
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