Julian Assange has suffered “prolonged exposure to psychological torture”, the UN’s torture expert has said.
Nils Melzer urged Britain not to extradite the WikiLeaks founder, warning that his human rights would be violated and that he is not fit to stand trial.
He also accused “several democratic states” of a “concerted effort to break [Assange’s] will”.
The UK government said it “disagreed with a number of his observations”.
A justice ministry spokesperson said the UK did not participate in torture. Judges were independent of the government and anyone convicted had the right to appeal, the spokesman added.
Assange, 47, is fighting extradition to the US over charges related to leaking government secrets.
He sought political asylum in the Ecuadorian embassy in London in 2012, where he stayed until his arrest earlier this year.
Mr Melzer, who met Assange earlier in May, told the Washington Post that he was initially reluctant to get involved in the case, as he is not a fan of WikiLeaks and considered its founder to be a bad actor.
What did Nils Melzer say?
The UN’s special rapporteur on torture said that Assange had been subjected to sustained collective persecution – including threatening statements and incitement to violence against him.
“I’ve worked in many areas of war in my life, in situations of violence, and I’ve talked to victims of persecution around the world and I’ve seen very serious atrocities,” Mr Melzer told the BBC.
“But [what] I have never seen is that a single person has been deliberately isolated and, I would say, persecuted – not prosecuted, but persecuted – by several democratic states in a concerted effort to eventually break his will.”
He added that he believes Assange “has a very strong case, and a very reasonable fear, that if he gets extradited to the Unites States he has no chance to get a fair trial with the level of public and official prejudice that exists there for him”.
Mr Melzer added that, because of his treatment, his health was at serious risk.
“We could see that Assange showed all the symptoms that are typical for prolonged exposure to psychological torture,” he said.
Assange, he said, needs access to a psychiatrist who is “not part of the prison service – someone he can fully trust” – to avoid his health deteriorating further.
What is the latest with the Julian Assange case?
Assange is currently serving a 50-week sentence in Belmarsh Prison in south east London for bail violations.
He had been due to appear at a hearing at Westminster Magistrates’ Court on Thursday – the second in his extradition case. However, his lawyer Gareth Peirce said that he was “not very well”.
A spokesman for WikiLeaks later said that he had been moved to the medical ward in jail, adding that he had “dramatically lost weight” while in prison.
“Defence lawyer for Assange, Per Samuelson, said that Julian Assange’s health state last Friday was such ‘that it was not possible to conduct a normal conversation with him’,” he added.
The US Justice Department has charged Assange with receiving and publishing thousands of classified documents linked to the wars in Iraq and Afghanistan.
The US wants the UK to extradite him, but Assange has formally refused consent.
Earlier this month, Swedish prosecutors also reopened their investigation into rape allegations against Assange, which he denies.