The whistleblower, who is being held alongside some of the UK’s most infamous criminals ahead of his extradition hearing in February, could face a maximum prison sentence of 175 years under charges laid down by Washington.
Now his father John Shipton has warned his son is suffering mentally and physically in prison.
Mr Shipton, who is due to accept an award from a whistleblower advocacy group on behalf of his son, said it was "extraordinary" that Assange was being held in one of the nation’s most notorious prisons despite calls for his release from the United Nations.
"The only people who are breaking the law are the UK government and the Crown Prosecution Service," he said.
"I last visited Julian in August - he was a bit shaky, and is suffering from anxiety. He has lost a lot of weight. It is very distressing, and the intensity of his treatment has increased over the past year.
"He is being subjected to every sort of torment."
Mr Assange was arrested at the London’s Ecuadorian embassy in April, having sought asylum in the building for seven years.
He is wanted on 18 charges in the US, including violation of the espionage act.
Many of the charges relate to founding and working at WikiLeaks - which openly published sensitive and classified documents, including more than a million files leaked by US Army intelligence analyst Chelsea Manning.
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His removal from the embassy also triggered renewed calls for him to stand trial in Sweden – with the nation’s government reopening the case into Mr Assange over allegations of rape made in 2010.
However, Sweden’s call to extradite him was rejected by courts in the country, delaying any potential action and prompting home secretary Sajid Javid to approve the request of the US government pending his trial.
Human rights organisations have raised concerns over the duration of Assange’s incarceration, including his time spent in the Ecuadorean Embassy, where he was left unable to leave for fear of arrest.
In May, Nils Melzer, the UN’s Special Rapporteur on torture and other cruel, inhuman or degrading treatment or punishment, said he condemned the his imprisonment, as well as the “deliberate, concerted and sustained nature of the abuse inflicted on Mr Assange”.
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. The collective persecution of Julian Assange must end here and now.”
Additional reporting by Press Association