A United Nations official has urged Britain not to extradite Julian Assange to the United States, warning that the WikiLeaks founder would not receive a fair trial after years of being subjected to "psychological torture".

Professor Nils Melzer, the UN Special Rapporteur on torture, said Mr Assange had been deliberately exposed to severe forms of "cruel, inhuman or degrading" treatment or punishment.

Mr Assange lived inside the Ecuadorian embassy in London for almost seven years before being dragged out last month and sentenced to 50 weeks in jail for a bail violation.

He now faces an extradition request from the US to face claims of violating the US Espionage Act by publishing classified military and diplomatic documents in 2010.

Mr Assange was too ill to appear at a court hearing in London yesterday.

Prof Melzer, who visited Mr Assange in Belmarsh Prison in south London earlier this month, issued a strongly worded statement expressing "alarm" that the US Department of Justice had laid 17 new charges, which carry up to 175 years in prison.

"This may well result in a life sentence without parole, or possibly even the death penalty, if further charges were added in the future," he said.

Prof Melzer said he was "gravely concerned" about the limited frequency of visits by Mr Assange's lawyers and his lack of access to documents, making it impossible for him to prepare his defence in the complex legal proceedings "piling up" against him.

The Swedish authorities recently announced that they were reopening an investigation into a rape allegation, which Mr Assange has always denied.

Prof Melzer said Mr Assange had been transferred to the healthcare unit at Belmarsh because his psychological state had deteriorated.

"Since 2010, when Wikileaks started publishing evidence of war crimes and torture committed by US forces, we have seen a sustained and concerted effort by several states towards getting Mr Assange extradited to the United States for prosecution, raising serious concern over the criminalisation of investigative journalism in violation of both the US Constitution and international human rights law," he said.

"Since then, there has been a relentless and unrestrained campaign of public mobbing, intimidation and defamation against Mr Assange, not only in the United States, but also in the United Kingdom, Sweden and, more recently, Ecuador."

According to the UN official, this included an "endless stream" of humiliating, debasing and threatening statements in the press and on social media, and by senior political figures, and magistrates involved in proceedings against Mr Assange.

"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.

"It was obvious that Mr Assange's health has been seriously affected by the extremely hostile and arbitrary environment he has been exposed to for many years.

"Most importantly, in addition to physical ailments, Mr Assange showed all symptoms typical for prolonged exposure to psychological torture, including extreme stress, chronic anxiety and intense psychological trauma.

"The evidence is overwhelming and clear.

"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.

"By displaying an attitude of complacency at best, and of complicity at worst, these governments have created an atmosphere of impunity encouraging Mr Assange's uninhibited vilification and abuse."

Prof Melzer appealed to the British Government not to extradite Mr Assange to America or to any other state failing to provide reliable guarantees against his onward transfer to the US.

He told the Press Association that extraditing Mr Assange to the US would be a "grave violation" of his human rights because there was "no chance" of him receiving a fair trial.

He added: "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."