Diplomats are investigating reports that a British embassy staff member is set to go on trial in Iran.
A lawyer acting for chief political analyst Hossein Rassam, an Iranian, said he is facing trial for "acting against national security". The Foreign Office said it had still received no official confirmation from Tehran.
Nine embassy staff were arrested last weekend amid deteriorating relations with Iran following the disputed re-election of president Mahmoud Ahmadinejad and the street protests that followed.
Foreign Secretary David Miliband said he was "deeply concerned" about the two who remain in detention and confident they had not engaged in "improper or illegal behaviour".
The prospect of legal action was first raised by influential cleric Ayatollah Ahmad Jannati, head of Iran's powerful Guardian Council, who claimed in a sermon that British embassy staff had made confessions.
He said: "In these events, their embassy had a presence. Some people were arrested. Well, inevitably, they will be put on trial."
Lawyer Abdolsamad Khorramshi said his client was being held in Evin prison, but that he had not been able to meet with him or access details of the charges.
The latest developments will further raise diplomatic tensions between the two countries, with the Iranian authorities seeking to blame Britain for stirring anti-Government unrest.
Britain has criticised Iran's crackdown on protesters and there have already been tit-for-tat expulsions of diplomats.
Iran's Ambassador to London, Rasoul Movahedian, was summoned by the Foreign Office last night as part of a co-ordinated action across European Union countries.
He was told that the remaining detainees should be released as soon as possible and without charge.
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