A joint letter from over 100 doctors urges Australia to protect imprisoned WikiLeaks founder Julian Assange.

The group has written an open letter to Foreign Minister Marise Payne, calling for Mr Assange to be returned to Australia for urgent medical treatment.

Mr Assange is in Belmarsh Prison, a high-security London jail, awaiting a full hearing next year.

He is set to fight extradition to the US, where he faces 18 charges, including conspiring to commit computer intrusion.

The doctors say an Australian citizen’s survival could be endangered by a foreign government obstructing his healthcare.

“It is an even more serious matter for that citizen's own government to refuse to intervene, against historical precedent and numerous converging lines of medical advice,” the letter says.

“Should Mr Assange die in a British prison, people will want to know what you, minister, did to prevent his death.”

United Nations' Special Rapporteur on Torture Nils Melzer says Mr Assange has been psychologically tortured.

The doctors agree, and say it is unbelievable that they have to urge the Australian and UK governments to give treatment to a torture victim.

“If Australia believes in universal moral values of truth ahead of authoritarian regimes using fear and abusing legal process to silence journalists, it must act to protect Julian Assange, his life, and his health,” said Australian doctor and former Democrats NSW state politician Arthur Chesterfield-Evans.