Accusations of the psychological torture of Julian Assange have not been addressed, with no legal basis for leaving the WikiLeaks founder locked up in solitary confinement in a high security prison, a human rights expert has claimed.

Nils Melzer, United Nations special rapporteur on torture, said Mr Assange’s health is being “destroyed” as he remains in Belmarsh prison in London as the United States continues to try to extradite him.

Mr Assange does not have access to his lawyers and is prevented from preparing his legal case, said Mr Melzer.

The UN official, speaking about his book The Trial Of Julian Assange, said allegations that Mr Assange hacked sensitive information were based on fabricated evidence.

“There is no serious crime he is being accused of. There is no legal basis for keeping him in a high security prison,” Mr Melzer told a media briefing organised by the Foreign Press Association.

He said he declined to become involved when he was first approached, shortly after Mr Assange sought refuge in the Ecuadorian embassy in London, but acted after receiving medical reports.

Mr Melzer visited the WikiLeaks founder in prison, saying: “I did not expect to find torture. What I found shocked me.”

He claimed “mass violations” of Mr Assange’s human rights, which he added have not been properly addressed.

Facts surrounding the case have been “deliberately distorted”, claimed Mr Melzer, adding that the governments of the UK, US and Sweden have refused to engage with him constructively.

He claimed there was a “wall of silence” surrounding the treatment of Mr Assange, saying he has been shocked by the lack of response from the public.