WikiLeaks founder Julian Assange could find out on Monday whether he has won a reprieve in his last-ditch legal battle to avoid extradition from Britain to the United States.

The 52-year-old Australian is seeking permission to appeal against a ruling allowing him to be sent to face a US trial on espionage charges, after a long-running court saga.

Two London High Court judges handling Assange's request adjourned the case in March, asking US government lawyers to give "satisfactory assurances" about free speech protections and that he would not face the death penalty if convicted.

Those submissions are expected to be presented at a hearing on Monday, and the judges could rule immediately afterwards.

If successful, Assange will be able to go back to domestic UK courts.

If he loses, Assange could be swiftly extradited after a five-year legal battle that has pitted the Washington and London governments against free-speech campaigners.

Assange's only hope would then be to appeal to the European Court of Human Rights, which could order a stay on the extradition if it decides there are "exceptional circumstances".

It would also require London to accept the order. This is uncertain because of a separate dispute with the European court which blocked the government's plan to send asylum seekers to Rwanda.

Dozens of Assange supporters gathered outside the Royal Courts of Justice in central London early Monday, many wearing T-shirts bearing Assange's face.

"This man's life is at stake," 83-year-old sculptor Jenny West told AFP.

"He represents all other journalists, it's a pressing humanitarian situation," she added.

- 'Corrupt' -

Assange has been detained in the high-security Belmarsh Prison in London since April 2019.

He was arrested after spending seven years holed up in Ecuador's London embassy to avoid extradition to Sweden, where he faced accusations of sexual assault that were eventually dropped.

US authorities want to put the publisher on trial for divulging US military secrets about the wars in Iraq and Afghanistan.

Assange is accused of publishing some 700,000 confidential documents relating to US military and diplomatic activities, starting in 2010.

The United States has accused Assange under the 1917 Espionage Act, which his supporters warn mean he could be sentenced to 175 years in prison.

UK courts approved the extradition request after the United States vowed that Assange would not go to its most extreme prison, "ADX Florence", nor to subject him to the harsh regime known as "Special Administrative Measures".

His supporters have criticised the legal proceedings he has faced.

"It is abundantly clear of course that the process in the court in the United Kingdom is corrupt. The case is rigged against Julian," Kristinn Hrafnsson, WikiLeaks' editor-in-chief, told reporters last Wednesday.

Stella Assange said she hoped her husband would be present at Monday's hearing but added that she did not expect the judges to rule in his favour.

"I don't expect a rational outcome from the courts, I'm afraid to say," she said.

Assange's supporters say his health is fragile and the Council of Europe this week voiced concern about his treatment.

The United States indicted Assange multiple times between 2018 and 2020 but President Joe Biden has faced domestic and international pressure to drop the case filed under his predecessor Donald Trump.

Biden indicated recently that the United States was considering an Australian request to drop the charges.

"President Biden has the chance still to be the president who put an end to this, who acted in the interest of press freedom in journalism," said Rebecca Vincent, of Reporters Without Borders (RSF).