A supporter holds a placard reading "Don't shoot the messenger - free Assange" during a vigil held for WikiLeaks founder Julian Assange outside the Ecuadorian Embassy in London. Photo: AFP

WikiLeaks founder Julian Assange has suffered psychological torture from a defamation campaign and should not be extradited to the US where he would face a "politicized show trial," a UN human rights investigator said Friday.

Nils Melzer, the UN 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.

"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 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 US, his lawyer Gareth Peirce said. 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.

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.