The US government won an appeal Friday against a UK court ruling that had blocked extraditing Julian Assange, paving the way for the WikiLeaks founder to be sent from Britain.

The decision by the High Court in London could now see the extradition referred to interior minister Priti Patel for final approval, but Assange's legal team immediately said it will ask the UK Supreme Court to intervene.

The US challenged a January decision by a British magistrates' court that it would be "oppressive" to extradite the 50-year-old Australian to the US justice system due to his mental health and the risk of suicide.

Washington wants Assange to face trial for WikiLeaks' publication in 2010 of classified military documents relating to its wars in Afghanistan and Iraq.

Assange's fiancee, Stella Moris, with whom he has two small children, has spearheaded the campaign for his release, calling the case a battle for media freedom
Assange's fiancee, Stella Moris, with whom he has two small children, has spearheaded the campaign for his release, calling the case a battle for media freedom AFP / Tolga Akmen

A two-day hearing in October saw lawyers for the United States argue that a lower court judge had not given sufficient weight to other expert testimony about his mental state.

They also pointed to diplomatic assurances provided since the January decision that Assange would not be held in punishing isolation at a federal supermax prison, and would receive appropriate care.

Approving the appeal, two judges at the High Court in London accepted the new assurances, noting they were not unusual in such cases and "solemn undertakings offered by one government to another".

The US government is challenging a London court ruling not to extradite Julian Assange to face charges relating to WikiLeaks' 2010 publication of classified US military documents about the wars in Afghanistan and Iraq
The US government is challenging a London court ruling not to extradite Julian Assange to face charges relating to WikiLeaks' 2010 publication of classified US military documents about the wars in Afghanistan and Iraq AFP / Tolga Akmen

Summarising the decision, judge Timothy Holroyde said the district judge should have sought such guarantees herself before ruling.

He added the High Court was satisfied they ensured Assange would not face "special measures" in the US, would "receive appropriate clinical and psychological treatment" and be eligible for transfer to Australia if convicted.

"The court rejected various criticisms of those assurances which were argued on Mr Assange's behalf and was satisfied that the assurances were sufficient to meet the concerns which led to the district judge's decision," Holroyde said.

Assange is being held at Belmarsh high-security prison in southeast London
Assange is being held at Belmarsh high-security prison in southeast London AFP / Hollie Adams

The two-judge ruling ordered that the case be returned to the magistrates' court with a direction it be sent to Patel for a final determination on extraditing Assange.

If she approves, the Wikileaks' founder would be sent to the US within 28 days.

However, Assange's partner Stella Moris said they would "appeal this decision at the earliest possible moment", calling it a "grave miscarriage of justice".

"How can it be fair, how can it be right, how can it be possible, to extradite Julian to the very country which plotted to kill him?" she said, referring to claims about a CIA plan under the Trump administration.

For his supporters, Assange is a fearless campaigner for press freedom. For others, he was reckless with classified information and possibly endangered sources
For his supporters, Assange is a fearless campaigner for press freedom. For others, he was reckless with classified information and possibly endangered sources AFP / Niklas HALLE'N

Birnberg Peirce Solicitors, which represent Assange, said in a statement that "his lawyers will be seeking permission to appeal this decision to the Supreme Court", noting they have 14 days to do so.

The long-running case has become a cause celebre for free speech, with Assange's supporters arguing WikiLeaks has the same rights as other media to publish secret material in the public interest.

Speaking outside court, Moris said the case was a "vindictive prosecution against a journalist".

"This goes to the fundamentals of press freedom and democracy," she said, accusing Britain of acting on behalf of a foreign power.

Russia, suspected of hacking 2016 US presidential candidate Hillary Clinton's campaign and handing materials to WikiLeaks, called the ruling "shameful".

Foreign ministry spokeswoman Maria Zakharova called it "a political case against a journalist and public figure -- another manifestation of the cannibalistic worldview of the Anglo-Saxon alliance".

Amnesty International also questioned the US assurances over Assange's future treatment, calling them "inherently unreliable".

Pro-Assange supporters gathered outside the court, waving placards and demanding his immediate release.

The US government has indicted Assange on 18 charges relating to WikiLeaks' release of 500,000 secret files on the conflicts in Afghanistan and Iraq.

He could be jailed for up to 175 years in the US, although the exact sentence is difficult to estimate.

Assange has been in custody since 2019, despite having served a previous sentence for breaching bail conditions in a separate case.

While at the South American country's mission, he fathered two children with Moris, who is a member of his legal team.

British prison authorities last month gave the couple permission to marry at Belmarsh prison in southeast London.