Britain's High Court has granted US authorities permission to expand their grounds for appealing an earlier court decision to block the extradition of Julian Assange, where he is wanted on espionage charges.

District Court Judge Vanessa Baraitser ruled in January that Mr Assange was likely to self harm if held under harsh US prison conditions.

The US government is appealing that decision.

Clair Dobbin, a lawyer who represented US authorities during a High Court hearing on Wednesday, said Assange — who she said "orchestrated one of the largest thefts of data in history" — does not meet the threshold of being "so ill" that he cannot resist harming himself.

She said a decision not to prosecute or extradite an individual would require "a mental illness of a type that the ability to resist suicide has been lost".

She argued that Assange's condition did not come close to being of that nature, and he has not made serious attempts on his life before.

Ms Dobbin also sought to discredit evidence from Assange's psychiatric expert, a key witness, arguing that he misled Judge Baraitser by concealing the fact that the 50-year-old Australian had fathered two children during his time hiding in the Ecuadorian Embassy in London.

On Wednesday two judges agreed to grant the US authorities permission to expand their grounds of appealing Judge Baraitser's decision to block the extradition.

A full appeal hearing is expected in October.

Assange, wearing a dark face mask, listened in by video link from London’s high-security Belmarsh prison, where he has been held since 2019.

Outside the court, Assange’s partner, Stella Moris, described him as an "innocent man accused of practicing journalism".

"For every day that this colossal injustice is allowed to continue, Julian’s situation grows increasingly desperate," Ms Moris, who has two young children with Assange, told his supporters and reporters.

"Julian has been denied the love and affection of his family for so long. Julian and the kids will never get this time back.

"This shouldn’t be happening."

Corbyn attends protest to free Assange

A group of protesters, including Jeremy Corbyn, the former leader of Britain's opposition Labour Party, held placards reading "journalism is not a crime" and shouted "free Julian Assange!" to the beat of a drum as police looked on.

US prosecutors have indicted Assange on 17 espionage charges and one charge of computer misuse over WikiLeaks' publication of thousands of leaked military and diplomatic documents a decade ago.

The charges carry a maximum sentence of 175 years in prison.

In January, Judge Baraitser accepted evidence from expert witnesses that Assange had a depressive disorder and an autism spectrum disorder.

She agreed that US prison conditions would be oppressive, saying there was a "real risk" he would be sent to the Administrative Maximum Facility in Florence, Colorado, the highest security prison in the US

But she rejected defence arguments that Assange faces a politically motivated American prosecution that would override free-speech protections.

She said the US judicial system would give him a fair trial.

Supporters and lawyers for Assange argue that he was acting as a journalist and is entitled to First Amendment protections of freedom of speech for publishing documents that exposed US military wrongdoing in Iraq and Afghanistan.

Lawyers for the US government, however, have said the case is largely based on "his unlawful involvement" in the theft of the diplomatic cables and military files by US Army intelligence analyst Chelsea Manning.

Assange was arrested in London in 2010 at the request of Sweden, which wanted to question him about allegations of rape and sexual assault made by two women.

In 2012, Assange jumped bail and sought refuge inside the Ecuadorian Embassy in London, where he remained holed up for the next seven years.

Ecuador withdrew the asylum it had granted him in 2019 and he was then immediately arrested for breaching bail.

Sweden dropped the sex crimes investigations in November 2019 because so much time had elapsed.

Ms Dobbin, representing the US government, said on Wednesday that the need to scrutinise January's ruling was "substantially increased" given the "extraordinary lengths" Assange had already gone to in order to avoid extradition.

"He was willing to break the law and no cost was too great, both in terms of the cost of policing his being in the embassy and of course the cost to himself," she said.

The case continues and a full appeal hearing was scheduled for October 27.

AP