LONDON — A British court ruled on Monday that WikiLeaks founder Julian Assange can appeal a decision that would allow for his extradition to the United States, where he would face charges under the Espionage Act in connection with obtaining and publishing secret government documents.

The latest twist in the long-running case comes after a High Court decision last month that Assange could be extradited, a reversal of a lower-court decision in a legal battle that has turned on whether prison conditions in the United States during his detention would be too harsh for his mental health.

Lord Chief Justice Ian Burnett of the High Court, in announcing the ruling in a brief court appearance on Monday morning, endorsed a further appeal of the case to the Supreme Court on a narrow issue: The timing of when the United States provided assurances that Assange would be treated humanely in U.S. prisons.

The Biden administration gave those assurances to the British government when the case was already before the High Court, after a lower court had considered the case and ruled that U.S. prison conditions were too harsh to extradite Assange, citing his mental health and the risk that he could be driven to suicide.

The High Court did not endorse an appeal of the separate issue of whether those assurances were adequate and credible, which Assange’s legal team has also contested. Still, Stella Morris, Assange’s partner, claimed victory outside the Royal Courts in central London after the announcement.

“What happened in court today is precisely what we wanted to happen,” Morris said. “Make no mistake,” she added, “we won today in court.”

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In preparing to appeal the High Court’s decision to permit the extradition, Assange’s lawyers, in keeping with British court procedure, first asked the High Court to certify that an issue within its own ruling was a “point of law of general public importance” — essentially meaning it is worthy of the Supreme Court’s attention.

The High Court’s decision that the timing issue meets that standard makes it more likely that the British Supreme Court will agree to take up any appeal, and Assange will now ask it to block his extradition, said Barry J. Pollack, an American lawyer for Assange.

Even as Morris and Assange’s legal team described the day’s events as a victory, they noted that he remained in detention. “Julian has to be freed,” Morris said, flanked by dozens of Assange’s supporters who had gathered outside the court. “And we hope that this will soon end. We are far from achieving justice in this case.”

A spokesman for the U.S. Department of Justice declined to comment.

Even if the Supreme Court declines to hear Assange’s appeal or rules against him regarding the assurances about his treatment, he has other options. His legal team could appeal to the High Court on other challenges to the extradition case, and the government would have the final word before he is extradited.

Last year, a lower court judge rejected claims from Assange’s legal team that the case against him itself was illegitimate, but nevertheless blocked the extradition request on the grounds that Assange might be driven to suicide if he were held in the austere conditions of the highest-security prison in the United States, citing his mental health.

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The United States appealed that decision, and the Biden administration in October said that Assange would not be held in the U.S. prison system’s harshest conditions for high-security prisoners. If he were convicted, it added, it would back any request he might make to serve any sentence in his native Australia. Last month, the High Court said that it was satisfied by those late-coming assurances, opening the door for the extradition of Assange.

The charges against Assange center on the 2010 publication of diplomatic and military files on his website, WikiLeaks, after they were leaked by Chelsea Manning, a former Army intelligence analyst.

The hundreds of thousands of State Department cables and military files published by WikiLeaks revealed previously undisclosed information about civilian deaths in the wars in Iraq and Afghanistan, the scope and limits of evidence against Guantánamo detainees, and hidden diplomatic dealings, among other things.

The indictments, which were handed down during the Trump administration, accuse Assange of participating in a criminal hacking conspiracy both by offering to aid Manning in covering her tracks on a secure computer network and by more broadly encouraging hackers to steal secret material and send it to WikiLeaks.

Prosecutors have also accused Assange of violating the Espionage Act by soliciting and publishing information the government deemed secret, charges that could raise profound First Amendment issues. A conviction on those grounds could establish a precedent that such journalistic-style activities — a separate question from whether Assange himself counts as a journalist — may be treated as a crime in the United States.

“He should not face criminal prosecution and decades in prison for publishing truthful information of great public importance,” Pollack said.

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The trial is playing out in a British courtroom after Assange spent years holed up in the Ecuadorian Embassy in London, fleeing there in 2012 as he faced an investigation on accusations of sexual assault in Sweden. Those charges were eventually dropped.

He was ejected from the embassy in 2019, and on the same day, the United States unsealed an initial indictment against him on a hacking-related charge. Weeks later, he was charged under the Espionage Act, and has been detained in Belmarsh prison in London since 2019.Supporters of Assange at the courthouse waved placards with his face and the message “Free Assange” as they cheered the small victory, but lamented that it would be likely to draw out his legal process for additional months.

Rebecca Vincent, who has been monitoring the extradition hearing for Reporters Without Borders and was in the courtroom Monday, said the decision was a welcome one.

“This is positive because there is another point of review here in the U.K., there is another stage with the courts rather than politicians that could see extradition refused,” she said. “So we very much hope that is the outcome.”