Julian Assange has been granted leave to appeal his extradition from Britain to the United States, after London’s High Court ruled he could be prejudiced there because he is Australian.

The decision, made at a hearing on Monday, means he will remain at Belmarsh Prison pending a hearing at a later date.

Mr Assange is facing extradition to the US, where he has been charged with 18 criminal offences, including obtaining, receiving and disclosing classified information.

The justices granted him leave to appeal on two grounds — including that he could be prejudiced because of his nationality — but denied a third.

At Monday's hearing, Mr Assange's lawyers told the court their client would be "discriminated against" in the US judicial system because he was Australian.

Mr Assange, 52, was asking the High Court of England and Wales for permission to appeal against a decision that would see him handed to American authorities.

Mr Assange is running out of legal options in the UK. In fact, if his permission to appeal was not granted, authorities could legally have hauled him out of Belmarsh Prison and on to a plane immediately, although that was unlikely.

Monday's hearing lasted just over two hours. Early arguments were centred around how Mr Assange would be treated if he was extradited.

Mr Assange did not attend the hearing, with his lawyers saying he was too unwell.

After a hearing in February, two British justices hearing the case had asked for several assurances from the US legal team if they did clear him to be sent to America, including: 

• That Mr Assange would be permitted to rely on the US constitution's first amendment (which protects free speech)
• That Mr Assange would not be prejudiced at trial or sentence because of his nationality
• That death penalty would not imposed should Mr Assange be convicted

Several weeks later, the US provided them, although they came with one catch.

In a diplomatic note sent from the US embassy in London, it said while Mr Assange would be able to "seek to rely" upon the US constitution's provisions for free speech, "a decision as to the applicability of the first amendment is exclusively within the purview of the US courts".

On Monday, Mr Assange's barrister, Edward Fitzgerald KC, told the court in London that the US's assurance his client "could seek to rely" on the first amendment was "blatantly inadequate".

"Prejudice will occur at trial by reason of the applicant's Australian nationality," he said.

He also pointed out it would be impossible for the UK court to bind its American counterpart with a ruling.

"The US constitution is supreme," he said.

Acting for the United States, barrister James Lewis KC told the court the US's diplomatic note explicitly said Mr Assange would not be prejudiced in the country's legal system because of his nationality.

"We do rely heavily on the first line of that assurance," he said.

"In our opinion it does make it clear he will not be discriminated against because of his nationality."

The charges relate to material published on Mr Assange's WikiLeaks website in 2010.

That information — given to him by soldier-turned-whistleblower Chelsea Manning — detailed evidence of, among other things, war crimes committed by US forces in Iraq and Afghanistan.

Mr Assange was living in the United Kingdom at the time of the publication, and this hearing represents his final legal avenue in that jurisdiction to avoid extradition.

Mr Assange's legal team has previously said it will attempt to challenge the extradition in the European Court of Human Rights (ECHR), if it runs out of legal options in the UK.

A date for the appeal has not yet been set.

Outside court, Mr Assange's wife, Stella, said she'd spoken to him in the wake of the ruling and that he was "relieved".

"We spent a long time hearing the United States putting lipstick on a pig, but the judges did not buy it," she said.

She also urged the Australian government to increase efforts for a diplomatic solution to see him released from jail.

"Whatever it takes, he needs to be free," she said.

Earlier this year, US President Joe Biden revealed he was "considering" a request to drop the case.

Gabriel Shipton, Mr Assange's Brother, told the ABC he would soon travel from London to the US to lobby officials.

"The only way that this is going to come to a quick conclusion is through the Australian government's advocacy and them using their coercive leverage with the United States, to get Julian home," he said.