Filmmaker James Ricketson, who spent 15 months as a political prisoner in a Cambodian jail, has implored Prime Minister Scott Morrison to "pick up the phone" to his British counterpart to ensure Julian Assange does not die in prison.

There are growing fears for the psychical and mental health of the 48-year-old WikiLeaks founder, who is in a London prison fighting an extradition request to the United States, where he faces espionage charges relating to the release of classified files on the Iraq and Afghanistan wars.

Julian Assange is driven from the Ecuadorian embassy in London after his arrest in April. Credit:PA

In an open letter to Mr Morrison, Mr Ricketson has joined a "rising tide of voices" in support of Australian government intervention to bring Mr Assange back to Australia before full extradition proceedings in February.

"The evidence that Julian Assange is not being ‘treated fairly’ in accordance with UK law is now overwhelming, as is evidence of the psychological torture he is being subjected to in Belmarsh Prison," Mr Ricketson writes.