The Australian-born Assange, 52, is wanted in the US on 18 charges, nearly all under the Espionage Act, relating to WikiLeaks' mass release of secret US documents - the largest security breaches of their kind in US military history.
The High Court had in March granted him provisional permission to appeal on grounds that he might be discriminated against as a foreign national, but invited the US to submit assurances.
Julian Assange has been under house arrest, in Ecuador's London embassy and jail since 2010. (AP PHOTO)
After Monday's hearing, two senior judges said Assange's argument that he might not be able to rely on the US First Amendment right to free speech deserved a full appeal - which is unlikely to be held for months.
The news prompted cheering and singing from hundreds of supporters who had massed outside the court tying yellow ribbons to the iron railings, holding placards and chanting "Free, free Julian Assange!".
"As a family we're relieved but how long can this go on?" said Assange's wife Stella, who had been in court with his brother and father.
"The United States should read the situation and drop this case now. Now is the moment to do it."
Julian Assange's wife, Stella, says it is time for the US to drop the extradition case against him. (AP PHOTO)
She told his supporters the ruling marked a turning point.
Assange himself was not present, which his lawyer said was for health reasons. The US Justice Department declined to comment on a pending judicial matter.
Had Monday's ruling gone against him, Assange's team said he could have been on a plane to the US within 24 hours, ending more than 13 years of legal battles in Britain.
US prosecutors had told the court Assange could "seek to rely" upon the First Amendment protections granted to US citizens, and would not be discriminated against because of his nationality.
But his legal team said a US court would not be bound by this. "We say this is a blatantly inadequate assurance," Assange's lawyer Edward Fitzgerald told the judges.
The court also concluded that Assange's appeal should apply to all 18 counts, not only three, as lawyers for the US had argued. Fitzgerald did, however, accept a separate US assurance that Assange would not face the death penalty.
Assange's lawyers will turn to the European Court of Human Rights if extradition is granted. (AP PHOTO)
WikiLeaks released hundreds of thousands of classified US military documents on Washington's wars in Afghanistan and Iraq along with swathes of diplomatic cables.
In April 2010 it published a classified video showing a 2007 US helicopter attack that killed a dozen people in the Iraqi capital, Baghdad, including two Reuters news staff. US authorities say Assange's actions with WikiLeaks were reckless, damaged national security, and endangered the lives of agents.
His many global supporters call the prosecution a travesty, an assault on journalism and free speech, and revenge for causing embarrassment. Calls for the case to be dropped have come from human rights groups, media bodies and Australian Prime Minister Anthony Albanese, along with other political leaders.
Assange was first arrested in Britain in 2010 on a Swedish warrant over sex crime allegations that were later dropped. Since then, he has been variously under house arrest, holed up in Ecuador's embassy in London for seven years and, since 2019, held in the Belmarsh top security jail.
He married Stella there in 2022 and the couple have two young children.
with PA
