He accuses the UK, US and Sweden of a "consistent failure" to protect Assange's human rights - and Australia of a "glaring absence" where it should be helping one of its citizens.
But the Australian government has emphatically rejected any suggestion Australia is complicit in any torture of Assange, saying it is confident he is being treated appropriately in London's Belmarsh Prison, and is giving him "active and high level" consular assistance - including making representations on his behalf to prison authorities to make sure he gets medical treatment.
Nils Melzer, a Geneva-based former Red Cross lawyer and human rights expert who is now the UN special rapporteur on torture, spent four hours with Assange in Belmarsh in early May, assessing his psychological and mental state along with two medical specialists.
In a currently confidential report submitted to the British government on Monday, along with letters to the US, Swedish and Ecuadorian governments, Melzer concluded Assange "shows all the symptoms of someone exposed to prolonged psychological ill-treatment".
"The evidence is overwhelming and clear," Melzer said. "Mr Assange has been deliberately exposed, for a period of several years, to progressively severe forms of cruel, inhuman or degrading treatment or punishment, the cumulative effects of which can only be described as psychological torture.
"I condemn, in the strongest terms, the deliberate, concerted and sustained nature of the abuse inflicted on Mr Assange and seriously deplore the consistent failure of all involved governments to take measures for the protection of his most fundamental human rights and dignity."
Melzer said the ill treatment was a combination of the way Assange was confined, isolated and persecuted while inside the Ecuadorean embassy, especially in his last year there, along with death threats and public accusations, the prosecutions pursued against him and the public statements made by US government officials as to how he should be dealt with.
Torture did not just include active efforts, but also covers a situation where a State is "aware your behaviour will have these consequences and not doing anything about it", Melzer said.
"In 20 years of work with victims of war, violence and political persecution I have never seen a group of democratic States ganging up to deliberately isolate, demonise and abuse a single individual for such a long time and with so little regard for human dignity and the rule of law."
Melzer told The Sydney Morning Herald and The Age that in his work with the UN and before in the field with the Red Cross he had seen people in rendition for interrogation after 9/11, and prisoners of war who had been ill-treated on a daily basis.
"But [Assange] is really something I've never seen in 20 years," Melzer said. "I've seen atrocities in war areas that were physically more horrible but I've never seen a single person pursued so relentlessly and with so little foundation.
"[When I saw him] I immediately compared him to some of the graver cases in interrogation prisons in terms of his psychological reaction patterns. That's what alarmed me so much."
He said Assange's treatment was "very close to the intentional, purposeful infliction of coercive measures to try to break him".
Melzer said his visit on May 9 involved a three-hour psychological and physical assessment based on the "Istanbul Protocol", a standard manual for assessing torture victims around the world.
The assessment took place before WikiLeaks revealed, on Wednesday, that Assange had been moved to a prison hospital having "dramatically lost weight" and in such a state that "it was not possible to conduct a normal conversation with him".
Assange did not appear as expected by video link for a short court hearing on Thursday.
Melzer said he was not surprised Assange had taken a turn for the worse.
When he visited Assange had complained of not being able to eat properly or keep his food down - which could have a psychological or physical cause.
He appeared "extremely agitated and preoccupied," Melzer said. "He asked a lot of questions and he would jump around, he was so preoccupied with everything he can't even compute my answers any more.
"There were episodes of this, then he was part of the conversation as normal, then again he would enter into this agitated state. I have seen with other victims of psychological torture that would happen."
Melzer said Assange's immediate medical needs were being attended to at Belmarsh, which had what he called "correct" infrastructure, "not great but nothing torturous in that".
But Assange, unlike other prisoners, was exposed to multiple major pending legal proceedings with "so much political commotion", and was not being given enough time to talk to his lawyers and get updates on his case.
Assange is currently fighting extradition to the US on hacking and espionage charges that carry a potential life sentence, and also faces possible extradition to Sweden to face a rape allegation.
Assange needed access to legal documents and a computer to write notes to share with his lawyers, Melzer said.
"It's not about creating special treatment it's simply giving him the means he needs, and is entitled to, to prepare a proper defence. It would certainly help to alleviate part of the pressure which is already superhuman."
"I fear if that pressure is not alleviated soon it might escalate in terms of the psychological consequences - and I think that's what we're now seeing."
Melzer said he had seen no sign of Australian assistance for Assange.
"Australia is a glaring absence in this case. They're just not around, as if Assange was not an Australian citizen. That is not the correct way of dealing with that."
A spokesperson for the Department of Foreign Affairs and Trade said "we reject any suggestion by the UN Special Rapporteur on Torture that the Australian Government is complicit in psychological torture or has shown a lack of consular support for Mr Assange".
Australian consular officials have visited Assange twice in prison, in mid-April and again two weeks ago. They raised Assange's health concerns with prison authorities, and "these have been addressed". And they are also in close contact with his family.
After it was reported Assange had been taken to the hospital prison this week, the Australian government again got in contact with the prison to check on him.
"We are confident that Mr Assange is being treated appropriately in Belmarsh Prison. Mr Assange has advised us that he is being treated the same as other prisoners in Belmarsh," the spokesperson said. "We will continue to visit Mr Assange in prison, monitor and advocate for his health, welfare and equitable treatment, and closely follow his legal proceedings."
Nick Miller is Europe correspondent for The Sydney Morning Herald and The Age
Comment: Ron Paul has basically accused the US of attempting to kill Assange: