A British judge on Monday blocked Julian Assange's extradition to the United States to face espionage charges after deciding the WikiLeaks founder was at serious risk of suicide, sparking disappointment in Washington.

District Judge Vanessa Baraitser said the 49-year-old Australian publisher would have been "well aware" of the effects of his leaking of secret documents, actions that went "well beyond" the role of a journalist.

Julian Assange
After Sweden first issued an arrest warrant for Assange in 2010 over allegations of sexual assault, he sought asylum in Ecuador's embassy in London Photo: AFP / Daniel LEAL-OLIVAS

But she said his mental health would probably deteriorate in the US penal system "causing him to commit suicide".

Assange wiped his forehead as the decision was announced in the central London court and his fiancee Stella Moris burst into tears and was embraced by WikiLeaks editor-in-chief Kristinn Hrafnsson.

He is still wanted on 18 charges in the US -- with a maximum 175 years in jail -- relating to the 2010 release by WikiLeaks of 500,000 secret files detailing aspects of military campaigns in Afghanistan and Iraq.

Julian Assange
The judge said Julian Assange was a suicide risk if he was sent into US custody Photo: AFP / Daniel LEAL-OLIVAS

Moris, who has two young sons with Assange, said the ruling was only a "first step towards justice" and appealed to US President Donald Trump to "end this now" so their sons could see their father.

Assange's supporters began gathering early in the day, defying police warnings to disperse because of coronavirus restrictions.

They erupted in cheers and shouted "Free Assange!" on hearing the ruling, which follows more than a decade of legal controversies.

Assange trial
Assange's partner Stella Moris and WikiLeaks' editor-in-chief Kristinn Hrafnsson arrive at court Photo: AFPTV / Mathilde BELLENGER

Assange and his lawyers have long argued that the protracted case is politically motivated.

The judge disagreed and the US government, which has two weeks to lodge its grounds to appeal, said it would challenge the decision.

The US Justice Department in Washington said it was "extremely disappointed" in the decision but was encouraged that the judge had indicated Assange appeared to have a legal case to answer.

Mexico's President Andres Manuel Lopez Obrador, meanwhile, offered Assange political asylum.

Julian Assange
Assange and his legal team have long argued that the protracted case, which has become a cause celebre for media freedom, was politically motivated Photo: AFP / DANIEL LEAL-OLIVAS

Assange was remanded in custody until a bail hearing on Wednesday.

Any decision to block extradition should meet a high bar given Britain's treaty obligations, Baraitser said.

Assange supporters
Assange supporters cheer as court blocks extradition to the US Photo: AFPTV / Linda ABI ASSI

But she rejected US experts' testimony that Assange would be protected from self-harm, noting that others such as disgraced US financier Jeffrey Epstein had managed to kill themselves in custody despite wardens' supervision.

"For this reason I have decided extradition would be oppressive by reason of mental harm and I order his discharge," she said.

The US non-profit Freedom of the Press Foundation said the case against Assange was "the most dangerous threat to US press freedom in decades".

"The extradition request was not decided on press freedom grounds; rather the judge essentially ruled the US prison system was too repressive to extradite."

Fugitive US whistleblower Edward Snowden said he hoped the ruling would mark "the end" of the case against Assange.

Before the ruling, both Germany and a UN rights expert expressed concern over the human rights and humanitarian problems presented by the extradition.

Assange has a history of depression and a respiratory condition that makes him more vulnerable to Covid-19, which has infected several inmates at the high-security prison where he has been held in London.

He has also complained of hearing imaginary voices and music during his detention.

UN special rapporteur on torture Nils Melzer has urged Trump to pardon Assange, saying he is not "an enemy of the American people".

The prospect of a possible pardon from the outgoing US leader has gained ground following a slew of others granted to a number of Trump's political allies.

The UK hearing was told Trump had promised to pardon Assange if he testified Russia had hacked into the computer servers of the Democratic National Committee (DNC) during the 2016 election campaign.

WikiLeaks later published the emails, which proved politically damaging to Trump's Democratic rival Hillary Clinton before the vote.

Washington claims Assange helped intelligence analyst Chelsea Manning steal the 2010 documents before exposing confidential sources around the world.

In April 2019, Ecuador, by then ruled by right-wing President Lenin Moreno, revoked his citizenship. British police dragged Assange out of the embassy.

Copyright AFP. All rights reserved.