UK officials will continue to lobby Iran for information about the chief political analyst from the British embassy in Tehran.
The eighth of nine British embassy employees detained by Iranian authorities is to be released later, the UK foreign secretary has said.
The Iranian man's lawyer has said his client is being held in prison accused of acting against national security.
David Miliband told the BBC's Andrew Marr programme that "all of our efforts are now directed" at securing the release of the final worker being held.
But Foreign Secretary David Miliband has so far been unable to discuss the case with his Iranian counterpart.
But the lawyer for the man says he has been charged with acting against national security and is facing trial.
Iran claims Britain is fomenting discontent to undermine Iran's Islamic regime - a charge the UK denies.
Iran has accused Britain of fomenting discontent to undermine its regime.
Political trial
Mr Miliband said he had learned the "good news" of the eighth release during his daily telephone conversation with Britain's ambassador to Iran.
Iranian news agencies have said all but one of nine embassy staff arrested last weekend have been released, although the UK government claims two remain in custody.
"He was told by the deputy foreign minister that the eighth person would indeed be released today, that the papers had been signed, that there would not be a court process or charges," Mr Miliband said.
IRAN UNREST 12 June presidential election saw incumbent Mahmoud Ahmadinejad re-elected with 63% of voteMain challenger Mir Hossein Mousavi called for result to be annulled for electoral fraudStreet protests saw at least 17 people killed and foreign media restricted class="" href="/2/hi/uk_news/8116245.stm">Britain and Iran's fraught history
"That leaves one more in custody and all of our efforts are now directed towards getting that person out."
A Foreign Office spokeswoman could not confirm reports that the adviser had been charged, adding that the situation was "unclear and fluid".
Foreign powers
Mr Miliband has said he is "deeply concerned" about the situation but a phone call he had planned to have with his Iranian counterpart did not take place.
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 British are being backed by the European Union in a call for the immediate and unconditional release of both embassy employees still being held.
Tehran has repeatedly accused foreign powers - especially Britain and the US - of stoking unrest after the election - somethig they deny.
BBC world affairs correspondent Caroline Hawley says the hope is that collective pressure might help to prevent a political trial.
The embassy workers were arrested last weekend amid accusations they had been involved in the demonstrations.
The lawyer, Abdolsamad Khorramshahi, claimed the man had been charged and would stand trial shortly.
"In these incidents, their embassy had a presence, some people were arrested," senior Iranian cleric Ahmad Jannati reportedly told the thousands of worshippers at Friday prayers.
'Confessions'
Mr Khorramshahi had not been able to meet his client in Evin prison where he is being held or see the text of the indictment, our correspondent said.
On Friday, Ahmad Jannati, leader of Iran's supreme legislative body the Guardian Council, said some members of the embassy staff would "inevitably" be put on trial and that they had made confessions.
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.
Tehran has repeatedly accused foreign powers - especially the UK 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.
