GENEVA, May 31 (Reuters) - WikiLeaks founder Julian Assange has suffered psychological torture from a defamation campaign and should not be extradited to the United States where he would face a "politicized show trial," a U.N. human rights investigator said on Friday.

Nils Melzer, the U.N. special rapporteur on torture who visited Assange in a high-security London prison on May 9 along with two medical experts, said that he found him agitated, under severe stress and unable to cope with his complex legal case.

"Our finding was that Mr. Assange shows all the symptoms of a person who has been exposed to psychological torture for a prolonged period of time. The psychiatrist who accompanied my mission said that his state of health was critical," Melzer told Reuters in an interview in Geneva.

"But my understanding is that he has now been hospitalized and that he is not able to stand trial," he said.

Assange was too ill on Thursday to appear via video link from a British prison in a hearing on an extradition request from the United States, his lawyer Gareth Peirce told Reuters. He is in a health ward.

"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," Melzer said in a statement.

The Swiss law professor declined to identify judges or senior politicians whom he accused of defaming Assange, saying "dozens if not hundreds of individuals" had expressed themselves inappropriately.

"Here we are not speaking of prosecution but of persecution. That means that judicial power, institutions and proceedings are being deliberately abused for ulterior motives," he added.

British Foreign Minister Jeremy Hunt, in a tweet posted within minutes of Melzer's statement, said: "This is wrong. Assange chose to hide in the embassy and was always free to leave and face justice.

"The UN Special Rapporteur should allow British courts to make their judgements without his interference or inflammatory accusations," he said.

14 PHOTOS Julian Assange See Gallery Julian Assange WikiLeaks founder Julian Assange makes a speech from the balcony of the Ecuadorian Embassy, in central London, Britain February 5, 2016. REUTERS/Peter Nicholls/Files A supporter of Wikileaks founder Julian Assange holds a banner outside the Ecuadorian embassy in London as he marks three years since Assange claimed asylum in the embassy on June 19, 2015. WikiLeaks founder Julian Assange clocks up three years inside the Ecuadoran embassy in London today, after claiming that Swedish prosecutors cancelled a landmark meeting in his case earlier this week. AFP PHOTO / JUSTIN TALLIS (Photo credit should read JUSTIN TALLIS/AFP/Getty Images) A supporter of Wikileaks founder Julian Assange holds banners outside the Ecuadorian embassy in London as he marks three years since Assange claimed asylum in the embassy on June 19, 2015. WikiLeaks founder Julian Assange clocks up three years inside the Ecuadoran embassy in London today, after claiming that Swedish prosecutors cancelled a landmark meeting in his case earlier this week. AFP PHOTO / JUSTIN TALLIS (Photo credit should read JUSTIN TALLIS/AFP/Getty Images) Julian Assange, Founder and Editor-in-Chief of WikiLeaks speaks via video link during a press conference on the occasion of the ten year anniversary celebration of WikiLeaks in Berlin, Germany, October 4, 2016. REUTERS/Axel Schmidt TPX IMAGES OF THE DAY WikiLeaks founder Julian Assange appears on screen via video link during his participation as a guest panelist in an International Seminar on the 60th anniversary of the college of Journalists of Chile in Santiago, Chile, July 12, 2016. REUTERS/Rodrigo Garrido File photo dated 05/02/16 of WikiLeaks founder Julian Assange, who will publish more confidential documents on the US Central Intelligence Agency once a "key attack code" has been disarmed, he has revealed. File photo dated 5/2/2016 of Julian Assange who has defended the release of emails by WikiLeaks about US presidential candidate Hillary Clinton. WikiLeaks founder Julian Assange speaking from the balcony of the Ecuadorian Embassy in London where he has been living for more than three years after the country granted him political asylum. BERLIN, GERMANY - OCTOBER 4: WikiLeaks founder Julian Assange participates via video link at a news conference marking the 10th anniversary of the secrecy-spilling group in Berlin, Germany on October 4, 2016. (Photo by Maurizio Gambarini/Anadolu Agency/Getty Images) The Spanish journalist Ignacio Ramonet presents in Quito, Ecuador, on June 23, 2016 the Ecuador 's book " When Google found Wikileaks". Julian Assange made his appearance to the world in 2010 with the publication by WikiLeaks of thousands of secret documents revealing conspiracies , corruption, crimes , lies, and incriminate several governments and particularly the United States as the main actor illegalities. (Photo by Rafael Rodrï¿½uez/ACGPHOTO/NurPhoto via Getty Images) LONDON, ENGLAND - FEBRUARY 05: Wikileaks founder Julian Assange prepares to speak from the balcony of the Ecuadorian embassy where he continues to seek asylum following an extradition request from Sweden in 2012, on February 5, 2016 in London, England. The United Nations Working Group on Arbitrary Detention has insisted that Mr Assange's detention should be brought to an end. (Photo by Carl Court/Getty Images) LONDON, ENGLAND - FEBRUARY 05: A panel of WikiLeaks representitives and press look on as WikiLeaks founder Julian Assange speaks at a press conference at the Frontline Club via video link from the Ecuadorian Embassy in London on 5 February 2016 in London, England. Mr Assange's speech comes a day after it was announced that the UN panel ruled he was being unlawfully detained at the Ecuadorian Embassy. (Photo by Chris Ratcliffe/Getty Images) Australian founder of whistleblowing website, 'WikiLeaks', Julian Assange speaks to media after giving a press conference in London on July 26, 2010. The founder of a website which published tens of thousands of leaked military files about the war in Afghanistan said Monday they showed that the 'course of the war needs to change'. In all, some 92,000 documents dating back to 2004 were released by the whistleblowers' website Wikileaks to the New York Times, Britain's Guardian newspaper, and Germany's Der Spiegel news weekly. Assange also used a press conference in London to dismiss the White House's furious reaction to the disclosures. AFP PHOTO/Leon Neal (Photo credit should read LEON NEAL/AFP/Getty Images) MELBOURNE, AUSTRALIA - MAY 21: (AUSTRALIA OUT) Wikileaks founder Julian Assange poses during a portrait shoot on May 21, 2010 in Melbourne, Australia. (Photo by Mark Chew/Fairfax Media/Fairfax Media via Getty Images). Up Next See Gallery Discover More Like This HIDE CAPTION SHOW CAPTION of SEE ALL BACK TO SLIDE

"POLITICIZED SHOW TRIAL"

Assange made headlines in 2010 when WikiLeaks published a classified U.S. military video showing a 2007 attack by Apache helicopters in Baghdad that killed a dozen people, including two Reuters news staff.

Washington is seeking the extradition of Assange, who was dragged from the Ecuadorean embassy in London on April 11 after his seven-year asylum was revoked, for one of the biggest ever leaks of classified information.

The Australian, now 47, had skipped bail and taken refuge there in 2012 to avoid extradition to Sweden over a sexual assault investigation later dropped. Sweden reopened the investigation in early May. Assange denies the rape allegation.

The United States has charged Assange with espionage, saying he unlawfully published the names of classified sources and conspired with and assisted ex-Army intelligence analyst Chelsea Manning in obtaining access to classified information. He faces 18 U.S. criminal counts and decades in prison if convicted.

"I am seriously, gravely concerned that if this man were to be extradited to the United States, he would be exposed to a politicized show trial and grave violations of his human rights," Melzer said.

"The main narrative in this affair really is the United States wanting to make an example of Mr. Assange in order to deter other people from following his example," he said.

Melzer did not expect U.S. authorities to subject Assange to physical torture such as water-boarding during interrogations.

"I would much more expect him to be subjected to prolonged solitary confinement, to very harsh detention conditions and to a psychological environment which would break him eventually."

(Reporting by Stephanie Nebehay Editing by Ros Russell)

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