Julian Assange is showing all the symptoms associated with prolonged exposure to psychological torture and should not be extradited to the US, according to a senior UN expert who visited him in prison.

Nils Melzer, UN’s special rapporteur on torture, is expected to make his appeal to the UK government on Friday. It comes after Assange, the co-founder of WikiLeaks, was said by his lawyers to be too ill to appear by video link for the latest court hearing of the case on Thursday.

Assange has been moved to the health ward of Belmarsh prison, London, where he has been serving a 50-week sentence for skipping bail while fighting extradition to the US. He is accused of violating the Espionage Act by publishing secret documents containing the names of confidential US military and diplomatic sources.

After meeting Assange earlier this month in the company of medical experts who examined him, Melzer will say on Friday that he fears the Australian’s human rights could be seriously violated if he is extradited to the US and will condemn what he describes as the “deliberate and concerted abuse inflicted for years” on him.

Assange was arrested in April after Ecuador revoked his political asylum and invited police inside the country’s Knightsbridge diplomatic premises, where he had sought refuge in 2012 to avoid extradition to Sweden over allegations of sexual assault, which he has denied.

“Physically there were ailments but that side of things are being addressed by the prison health service and there was nothing urgent or dangerous in that way,” Melzer said.

“What was worrying was the psychological side and his constant anxiety. It was perceptible that he had a sense of being under threat from everyone. He understood what my function was but it’s more that he was extremely agitated and busy with his own thoughts. It was difficult to have a very structured conversation with him.”

Melzer said that Belmarsh was an old prison and had issues about that but he described it as well maintained, adding that characterisations of it as a “supermax” or “the Guantanamo of Britain” were unhelpful. While it does have a high-security wing Melzer said that Assange was not in that section.

The lawyer, who receives 10 to 15 requests each day from sources asking for him to get involved, said that his office had been approached by Assange’s lawyers in December. But he said that he was initially reluctant to do so, admitting he was affected by what he called the “prejudice” around the case.

However, he began looking into the case again in March and, earlier this week, wrote letters to the foreign ministers of the US, the UK and Sweden.

“In the course of the past nine years, Mr Assange has been exposed to persistent, progressively severe abuse ranging from systematic judicial persecution and arbitrary confinement in the Ecuadorian embassy, to his oppressive isolation, harassment and surveillance inside the embassy, and from deliberate collective ridicule, insults and humiliation, to open instigation of violence and even repeated calls for his assassination,” Melzer will say on Friday.

He added the UK authorities had contacted his Geneva office to indicate that the British government would be issuing a point-by-point rebuttal of the assertions made in his letter.

Melzer, who is urging the UK government not to extradite Assange to the US or to any other state failing to provide reliable guarantees against his onward transfer to the US, criticised the way in which Assange’s case was handled after police took him from the embassy.

“I was surprised, for example, to see that on the date he was arrested he was immediately brought to court after six years in the embassy and then convicted. Under the normal rule of law you would expect someone to be arrested and then given a couple of weeks to prepare his defence at least.”

The former legal adviser to the Red Cross will say on Friday: “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.”

Assange could face decades in a US prison after being charged with violating the Espionage Act by publishing classified information through WikiLeaks.

Prosecutors earlier this month announced 17 additional charges against him for publishing hundreds of thousands of secret diplomatic cables and files on the wars in Afghanistan and Iraq.

The 47-year-old was previously charged with working to hack a Pentagon computer system, in a secret indictment that was unveiled soon after his arrest at Ecuador’s embassy in London.

“He’s in fact far from well,” Assange’s lawyer, Gareth Peirce told the hearing on Thursday at the Westminster magistrates court. The next hearing on the extradition request was set for 12 June.

A UK government spokesperson said: “The UK has a close working relationship with UN bodies and is committed to upholding the rule of law. We support the important work of the special rapporteur’s mandate and will respond to his letter in due course, but we disagree with a number of his observations.

“Judges are impartial and independent from government, with any judgment based solely on the facts of the case and the applicable law. The law provides all those convicted with a right of appeal.”