Iran faces mounting pressure from the UK to free British embassy workers being held by the authorities.
The Foreign Office has not been able to verify whether one of the last two remaining behind bars had been set free, after receiving assurances they would be released.
Foreign Secretary David Miliband launched a vigorous defence of the other employee, believed to be the embassy's chief political analyst Hossein Rassam, who is said by his lawyer to face trial for "acting against national security".
Nine staff were originally arrested after the disputed re-election of president Mahmoud Ahmadinejad and the street protests that followed.
Amid mounting diplomatic tensions, Mr Miliband accused the Iranian regime of pursuing a "grim and gruesome" clampdown against protesters angry at the re-election of Mr Ahmadinejad.
Tehran publicly accuses the UK of fomenting the unrest and a senior cleric said on Friday that some embassy staff, all Iranian, had "confessed" to playing a part and would face trial.
But Mr Miliband rejected the allegations. He said: "The allegations of improper conduct have absolutely no basis."
He said the individual was "an honourable, patriotic Iranian, who has been working in a completely open and transparent way for the UK".
"I think it is very, very important that we send a clear message that we are confident about the way he has been doing his job, that we are clear about our goal, which is his release, unharmed, and also that there is unity across the international community," he went on.
"It is very important that my anger, my cold anger about the way our staff have been treated, in this case Iranian citizens, doesn't turn into a rhetorical volley at the Iranian regime because that doesn't do anything either for our people or for reform in Iran," he said. "What's important is that I turn my anger into determination to see that justice is done by our people."
