Iran has said it will release the eighth of nine British embassy employees detained in Tehran today, Foreign Secretary David Miliband revealed.
But he vowed to continue to fight for the freedom of the remaining detainee who is facing a possible trial on charges Mr Miliband insisted had "absolutely no substance".
The staff were arrested last weekend amid rapidly-deteriorating relations with Iran since the disputed re-election of president Mahmoud Ahmadinejad and the street protests that followed.
"I spoke to our ambassador on our daily call last night and the good news is that he was told by the deputy foreign minister that the eighth person would indeed be released today, that the papers have been signed and that there would not be a court process or charges.
"That leaves one more in custody and all of our efforts are now directed towards getting that person out," he told BBC1's Andrew Marr Show.
The embassy's chief political analyst, Hossein Rassam, an Iranian, was identified by his lawyer as the individual who remained in detention facing a charge of "acting against national security".
Mr Miliband said he felt "cold anger" about the treatment of the staff, who are all Iranian, but would channel his emotion into efforts to secure the release of the last remaining detainee..
"The allegations of improper conduct have absolutely no basis," he said, saying 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."