The US has asked the High Court to overturn the decision that blocked Julian Assange’s extradition on spying charges.
In January, a judge said the WikiLeaks founder should not be handed over as he might kill himself in a US prison.
US authorities have now said Assange could serve any jail term in his home country of Australia.
James Lewis QC, a lawyer for the US government, said on Wednesday that the judge in the earlier case, Vanessa Baraitser, had made an error.
He said authorities had promised that Assange wouldn’t be held in harsh conditions such as a “supermax” prison or in isolation, and that any jail term following a trial could be served in Australia.
Mr Lewis said the assurances were “binding”.
The US case also argues that Assange does not meet the bar of being so unwell that he could not resist harming himself.
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“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,” Mr Lewis said.
A large group of supporters of Assange gathered outside the hearing at the Royal Courts of Justice, including Chinese artist Ai Weiwei.
Assange, 50, is currently being held at the UK’s Belmarsh maximum-security prison.
He watched via video link from the southeast London jail and appeared to wear a black face covering and a burgundy tie.
Stella Moris, Assange’s partner, said outside the London court that she was “very concerned for Julian’s health”, saying she had seen him at the weekend and that he was “very thin”.
“It is completely unthinkable that the UK courts could agree to this,” she said. “I hope the courts will end this nightmare, that Julian is able to come home soon and that wise heads prevail.”
England’s most senior judge, Lord Chief Justice Ian Burnett, is among those hearing the appeal, with a ruling not expected for several weeks.
Assange is alleged to have helped US Army intelligence analyst Chelsea Manning steal classified diplomatic cables and military files exposing wrongdoing in Iraq and Afghanistan that WikiLeaks later published
Prosecutors in the US have indicted him on 17 espionage charges and one of computer misuse which carry a maximum 175-year sentence.
Assange’s legal team say as a journalist he is entitled to First Amendment freedom of speech protection.
The Australian was arrested in April 2019 after spending seven years under diplomatic protection in Ecuador’s London embassy.