Julian Assange has been “psychologically tortured” and should not be extradited to the US, a United Nations official has said.

Nils Melzer, the UN special rapporteur on torture, said the WikiLeaks founder would not receive a fair trial after visiting him in a British prison.

He said that although Assange was not being held in solitary confinement at HMP Belmarsh, limited legal visits and a lack of access to documents “make it impossible for him to adequately prepare his defence”.

“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,” Mr Melzer said.

“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.”

Key moments for Julian Assange Show all 9 1 /9 Key moments for Julian Assange Key moments for Julian Assange The situation today Assange was arrested after Metropolitan Police officers were invited into the Ecuadorian embassy on April 11 2019. How did it come to this? Ruptly TV Key moments for Julian Assange The break Assange shows the front page of the Guardian on July 26 2010, the day that they broke the story of the thousands of military files leaked by WikiLeaks AFP/Getty Key moments for Julian Assange Wanted A warrant for Assange's arrest was issued in August 2010 for counts of rape and molestation in Sweden AFP/Getty Key moments for Julian Assange Ruling The UK's Supreme Court ruled in 2012 that Assange should be extradited to Sweden to face trial Getty Key moments for Julian Assange Sanctuary Following the ruling, Assange was given asylum by the Ecuadorian governement over fears that his human rights would be violated if he were extradited, he has since remained in the embassy in London Getty Key moments for Julian Assange A friend in Pam Friend Pamela Anderson delivers lunch to Assange at the embassy in October 2016. She has since spoken against his arrest Getty Key moments for Julian Assange Arbitrarily detained A UN panel found in 2016 that Assange had been arbitrarily detained and that he had not been able to claim his full right to asylum. It urged Sweden to withdraw the charges against him Getty Key moments for Julian Assange The cat ultimatum Last year, the Ecuadorian embassy threatened to revoke Assange's internet access unless he stopped making political statements online and started taking better care of James, his pet cat. Assange accused Ecuador of violating his rights Reuters Key moments for Julian Assange Arrest Assange was arrested on April 11 2019. Ecuador revoked his asylum status and invited the Metropolitan Police in to the embassy to arrest him. Reuters

He was accompanied during his prison visit on 9 May by two medical experts, who found that Assange’s health had been “seriously affected by the extremely hostile and arbitrary environment he has been exposed to for many years”.

They found that Assange showed symptoms of extreme stress, chronic anxiety and intense psychological trauma.

The strongly-worded statement came a day after Assange was unable to attend a court hearing because of ill health.

Westminster Magistrates’ Court heard that the Australian detainee was “not very well” and had been moved to the prison’s medical ward.

A judge scheduled another hearing on 12 June and said it may take place inside HMP Belmarsh, where Assange is serving a 50-week prison sentence for breaking bail conditions.

He fled to the Ecuadorian embassy in 2012, after losing a legal battle against extradition to Sweden for a rape investigation.

But Ecuadorian authorities revoked Assange’s political asylum in April and invited British police to arrest him, before issuing statements accusing him of violating international conventions and unacceptable conduct.

Swedish authorities reopened its probe after Assange was arrested and may attempt to have him extradited a second time to face the allegations, which he denies.

Sweden’s deputy director of public prosecutions said a court will consider “detaining Assange in his absence” on Monday.

Julian Assange: Swedish prosecutors reopen investigation into rape allegation made against Wikileaks co-founder

“If the court decides to detain him, I will issue a European arrest warrant concerning his surrender to Sweden,” Eva-Marie Persson added.

“In the event of a conflict between a European arrest warrant and a request for extradition from the US, UK authorities will decide on the order of priority. The outcome of this process is impossible to predict.”

The US Department of Justice initially announced that Assange was charged with conspiracy to commit computer intrusion by helping Chelsea Manning crack a password for government systems.

The breach resulted in WikiLeaks publishing a slew of leaked diplomatic cables, battlefield reports and footage from Iraq and Afghanistan that included evidence of war crimes and torture in 2010.

The US Department of Justice filed 17 more charges against Assange last week, accusing him of violating espionage laws by seeking, obtaining and disseminating classified information that “could harm the national security of the United States”.

Mr Melzer said he was “alarmed” by the new charges and said they could result in a life sentence without parole “or possibly even the death penalty” if further allegations are added.

“My most urgent concern is that in the United States, Mr Assange would be exposed to a real risk of serious violations of his human rights,” he added.

The UN official claimed that since 2010, there has been a “relentless and unrestrained campaign of public mobbing, intimidation and defamation” against Assange in the US, UK, Sweden and by Ecuadorian officials.

“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,” Mr Melzer said.

Pro-Assange protesters outside Westminster Magistrates’ Court today (AP)

“The collective persecution of Julian Assange must end here and now.”

Jeremy Hunt, the British foreign secretary, said Mr Melzer’s assessment was “wrong”.

“Assange chose to hide in the embassy and was always free to leave and face justice,” he wrote on Twitter.

“The UN special rapporteur should allow British courts to make their judgements without his interference or inflammatory accusations.”

In official letters sent earlier this week, Mr Melzer urged the US, British, Swedish and Ecuadorian governments to stop statements and activity “prejudicial to Assange’s human rights and dignity”.

He directly appealed to the British government not to extradite Assange to the US or any country that would allow such a transfer.

The UN said the UK was “reminded of its obligation to ensure Assange’s unimpeded access to legal counsel, documentation and adequate preparation commensurate with the complexity of the pending proceedings”.

Britain’s Ministry of Justice has denied giving Assange exceptional treatment and said that although HMP Belmarsh is notorious for holding terrorists and murders, it also serves as a low-security “local prison” for people awaiting trial and those on short sentences like Assange’s.

Nationwide rules state that prisoners cannot access the internet other than for educational or resettlement purposes, and cannot be emailed directly.

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.