US authorities have told British judges that if they agree to extradite Julian Assange on espionage charges, the WikiLeaks founder could serve any US prison sentence he receives in his native Australia.

In January, a lower UK court refused a US request to extradite Assange over WikiLeaks’ publication of secret US military documents a decade ago.

District Judge Vanessa Baraitser ruled that Assange, who has spent years in hiding and in UK prisons as he fights extradition, was likely to take his own life if held under harsh US prison conditions.

Appealing against that decision at the high court in London, a lawyer for the US government on Wednesday denied Assange’s mental health was too fragile to withstand the US judicial system.

Lawyer James Lewis said Assange “has no history of serious and enduring mental illness” and does not meet the threshold of being so ill that he cannot resist harming himself.

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.

The charges carry a maximum sentence of 175 years in prison, although Lewis said “the longest sentence ever imposed for this offence is 63 months”.

Lewis said US authorities had promised Assange would not be held before trial in a top security “Supermax” prison or subjected to strict isolation conditions, and if convicted would be allowed to serve his sentence in Australia.

Lewis said the assurances “are binding on the United States”.

“Once there is an assurance of appropriate medical care, once it is clear he will be repatriated to Australia to serve any sentence, then we can safely say the district judge would not have decided the relevant question in the way that she did,” he said.

The US also says a key defence witness, neuropsychiatrist Michael Kopelman, misled the previous judge by omitting to mention that Stella Moris, a member of WikiLeaks’ legal team, was also Assange’s partner and had two children with him.

Lewis said that information was “a highly relevant factor to the question of likelihood to suicide”.

Assange’s lawyer Edward Fitzgerald accused US lawyers of seeking to “minimise the severity of Mr Assange’s mental disorder and suicide risk”.

Fitzgerald said in a written submission Australia had not yet agreed to take Assange if he was convicted.

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Even if Australia did agree, Fitzgerald said the US legal process could take a decade, “during which Mr Assange will remain detained in extreme isolation in a US prison”.

Assange, who is being held at London’s high-security Belmarsh prison, had been expected to attend the two-day hearing by video link but Fitzgerald said Assange had been put on a high dose of medication and “doesn’t feel able to attend”.

Assange later appeared on the video link at times, seated at a table in a prison room wearing a black face mask.

Since WikiLeaks began publishing classified documents more than a decade ago, Assange has become a flashpoint figure.

Some see him as a dangerous secret-spiller who endangered the lives of informers and others who helped the US in war zones.

Others say WikiLeaks shone a light on official malfeasance that governments would like to keep secret.

Several dozen pro-Assange protesters held a boisterous rally outside London’s Royal Courts of Justice on Wednesday, calling the prosecution politically motivated.

WikiLeaks supporters say testimony from witnesses during the extradition hearing that Assange was spied on while in Ecuador’s embassy in London by a Spanish security firm at the behest of the CIA – and that there was even talk of abducting or killing him – undermines US claims he will be treated fairly.

The two justices hearing the appeal – one is England’s most senior judge, lord chief justice Ian Burnett – are not expected to give their ruling for several weeks.

The losing side could seek to appeal to the UK supreme court.

Outside court, Moris said it was “completely unthinkable that the UK courts could agree” to extradition.

“I hope the courts will end this nightmare, that Julian is able to come home soon and that wise heads prevail,” she said.