More than four years since Assange moved into the Ecuadorian embassy to escape extradition to Sweden, where he faces a police investigation into rape claims, WikiLeaks argued that his confinement puts his health at risk.
"Mr Assange's mental health is highly likely to deteriorate over time if he remains in his current situation... It is urgent that his current circumstances are resolved as quickly as possible," said a report published by the organisation on Twitter.
It comes ahead of a decision expected on Friday by a Swedish appeals court on whether to maintain an arrest warrant for Assange over a 2010 rape accusation.
The medical document is said to be based on five interviews carried out in the year to June 2015 and includes comments from others at the embassy.
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According to the report, Assange often goes up to 22 hours without sleeping. He has access to a treadmill for physical exercise.
"Since June 2015, however, his physical condition has deteriorated due to limited range of movement, inability to exercise normally and constant pain," the report added, quoted the unnamed expert.
One of Assange's colleagues was also quoted as saying in the report it had been difficult to find doctors willing to examine the WikiLeaks founder at the embassy.
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