US prosecutors are seeking to put Assange, 52, on trial on 18 counts relating to WikiLeaks' high-profile release of vast troves of confidential US military records and diplomatic cables.

They argue the leaks imperilled the lives of their agents and there is no excuse for his criminality. Assange's many supporters hail him as an anti-establishment hero and a journalist, who is being persecuted for exposing US wrongdoing and committing alleged war crimes.

Assange's legal battles began in 2010, and he subsequently spent seven years holed up in Ecuador's embassy in London before he was dragged out and jailed in 2019 for breaching bail conditions. He has been held in a maximum-security jail in southeast London ever since, even getting married there.

Britain finally approved his extradition to the US in 2022 after a judge initially blocked it because concerns about his mental health meant he would be at risk of suicide if deported.

His lawyers will try to overturn that approval at a two-day hearing in front of two judges at London's High Court in what could be his last chance to stop his extradition in the English courts. His wife Stella last week described it as a matter of life and death.

They will argue that Assange's prosecution is politically motivated and marks an impermissible attack on free speech, as the first time a publisher has been charged under the US Espionage Act.

His supporters include Amnesty International, Reporters Without Borders, media organisations which worked with WikiLeaks and Australian politicians, including Prime Minister Anthony Albanese, who last week voted in favour of a motion calling for his return to Australia.

Pope Francis even granted his wife an audience last year.

If Assange wins permission in the latest case, a full appeal hearing will be held to again consider his challenge. If he loses, his only remaining option would be at the European Court of Human Rights (ECHR) where he has an appeal already lodged pending the London ruling.

Speaking last week, Stella Assange said they would apply to the ECHR for an emergency injunction if necessary. She said her husband would not survive if he was extradited.

"His health is in decline, physically and mentally," she said. "His life is at risk every single day he stays in prison – and if he is extradited he will die."

Assange's brother Gabriel Shipton compared the WikiLeaks founder with Alexei Navalny, the Russian opposition activist who died in prison on Friday while serving a three-decade sentence.

"I know exactly what it feels like to have a loved one unjustly incarcerated with no hope," he told the BBC. "To have them pass away, that's what we live in fear of: that Julian will be lost to us, lost to the US prison system or even die in jail in the UK."

WikiLeaks first came to prominence in 2010 when it published a US military video showing a 2007 attack by Apache helicopters in Baghdad that killed a dozen people, including two Reuters news staff.

It then released thousands of secret classified files and diplomatic cables that laid bare often highly critical US appraisals of world leaders from Russian President Vladimir Putin to members of the Saudi royal family.

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