If a British court permits the extradition of the WikiLeaks founder Julian Assange to face criminal charges in the United States, the Biden administration has pledged that it will not hold him under the most austere conditions reserved for high-security prisoners and that, if he is convicted, it will let him serve his sentence in his native Australia.
Those assurances were disclosed on Wednesday as part of a British High Court ruling in London. The court accepted the United States government’s appeal of a ruling that had denied its extradition request for Mr. Assange — who was indicted during the Trump administration — on the grounds that American prison conditions for the highest-security inmates were inhumane.
The new ruling was not made public in its entirety. But in an email, the Crown Prosecution Service press office provided a summary showing that the High Court had accepted three of five grounds for appeal submitted by the United States and disclosing the promises the Biden administration had made.
A lower-court judge, Vanessa Baraitser of the Westminster Magistrates’ Court, had held in January that “the mental condition of Mr. Assange is such that it would be oppressive to extradite him to the United States” given American prison conditions. The summary of the decision to accept the appeal said that the United States had “provided the United Kingdom with a package of assurances which are responsive to the district judge’s specific findings in this case.”
Specifically, it said, Mr. Assange would not be subjected to measures that curtail a prisoner’s contact with the outside world and can amount to solitary confinement, and would not be imprisoned at the supermax prison in Florence, Colo., unless he later did something “that meets the test” for imposing such harsh steps.
“The United States has also provided an assurance that the United States will consent to Mr. Assange being transferred to Australia to serve any custodial sentence imposed on him,” the summary said.
No hearing date has been set. The Crown Prosecution Service and the United States Justice Department declined to comment.
In a statement, Stella Moris, Mr. Assange’s fiancée, urged the Biden administration to instead drop the extradition request and abandon the charges, which she portrayed as a threat to First Amendment press freedoms.
“I am appealing directly to the Biden government to do the right thing, even at this late stage,” she said. “This case should not be dragged out for a moment longer. End this prosecution, protect free speech and let Julian come home to his family.”
The case against Mr. Assange is complex and developed over the course of three indictments secured by prosecutors during the Trump administration. It centers on his 2010 publication of diplomatic and military files leaked by Chelsea Manning, a former Army intelligence analyst — not on his publication during the 2016 election of Democratic emails stolen by Russia.
Prosecutors have made two sets of accusations. One is that Mr. Assange participated in a criminal hacking conspiracy, both by offering to help Ms. Manning mask her tracks on a secure computer network and by engaging in a broader effort to encourage hackers to obtain secret material and send it to WikiLeaks. The other is that his soliciting and publishing information the government had deemed secret violated the Espionage Act.
While hacking is not a journalistic act, the second set of charges has alarmed press-freedom advocates because it could establish a precedent that such journalistic-style activities may be treated as a crime in the United States — a separate question from whether Mr. Assange himself counts as a journalist.
In January, Judge Baraitser rejected the Trump administration’s extradition request on the grounds that Mr. Assange might be driven to suicide by American prison conditions. On Jan. 19, in one of its last acts, the Trump administration filed an appeal of that ruling. Soon after taking office, the Biden administration pressed forward.
Elian Peltier contributed reporting from London.