David Miliband: "There is no place for this sort of intimidation or harassment"
The eighth of nine British embassy employees detained by Iranian authorities has been released, the UK foreign office says.
The last remaining detainee, an Iranian, is the embassy's chief political analyst. He has been charged with acting against national security.
UK PM Gordon Brown described the continued detention as "unacceptable and unjustified".
Meanwhile Iran's supreme leader warned the West not to "meddle".
"Some leaders of Western countries at the level of president, prime minister and foreign minister openly intervened in Iran's internal affairs that had nothing to do with them," Ayatollah Ali Khamenei was quoted as saying on Iranian state television.
"Then, they said they don't intervene in Iran's internal affairs."
'Honourable and patriotic'
Speaking at a news conference following a Franco-British summit in the French town of Evian, Mr Brown warned of concerted action against Iran.
"The Iranian regime must be clear that if this action continues and we are forced to act, we will act together with our European partners", he said.
It is very important that my cold anger... doesn't turn into a rhetorical volley at the Iranian regime
Foreign Secretary David Miliband said earlier that every effort would be made to secure the remaining detainee's release.
He said he had learned the "good news" of the eighth release during his daily telephone conversation with Britain's ambassador to Iran.
"[The ambassador] 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.
"That leaves one more in custody and all of our efforts are now directed towards getting that person out."
On Saturday, the man's lawyer, Abdolsamad Khorramshahi, said he had been unable to meet him in Evin prison in Tehran where he is being held, or see the text of the indictment.
Mr Miliband said the man was "an honourable, patriotic Iranian, who has been working in a completely open and transparent way for the UK".
"The allegations of improper conduct have absolutely no basis," he said.
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.
Tehran has repeatedly accused foreign powers, especially Britain and the US, of stoking unrest after the election - something they deny.
The embassy workers, who are all Iranian, were arrested last weekend amid accusations they had been involved in the demonstrations.
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.
Protests have taken place outside the British embassy in Tehran
"Well, inevitably they will be put on trial. They have made confessions too."
Mr Miliband said he was angry, but would try not to inflame the already sensitive situation further.
"It is very important that my anger, my cold anger, about the way our staff have been treated... 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."
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