Protesters hold up placards outside the Home Office building, in London, during a demonstration to protest against the extradition of Julian Assange on July 1, 2022.
The appeal was filed Friday at the High Court, the latest twist in a decade-long legal saga sparked by his website's publication of classified U.S. documents. No further details about the appeal were immediately available.
Assange's supporters staged protests before his 51st birthday on Saturday, with his wife Stella Assange among people who gathered outside the Home Office on Friday to call for his release from prison.
Julian Assange has battled in British courts for years to avoid being sent to the U.S., where he faces 17 charges of espionage and one charge of computer misuse.
American prosecutors say the Australian citizen helped U.S. Army intelligence analyst Chelsea Manning steal classified diplomatic cables and military files that WikiLeaks later published, putting lives at risk.
To his supporters, Assange is a secrecy-busting journalist who exposed U.S. military wrongdoing in Iraq and Afghanistan.
A British court ruled in April that Assange could be sent to face trial in the U.S., sending the case to the U.K. government for a decision. Home Secretary Priti Patel signed an order on June 17 authorizing Assange's extradition.
The Australian government has been under mounting pressure to intervene, but last month Prime Minister Anthony Albanese rejected calls for him to publicly demand that Washington drop its prosecution of Assange.
Assange's supporters and lawyers maintain he was acting as a journalist and is entitled to First Amendment protections of freedom of speech. They argue that the case is politically motivated, that he would face inhumane treatment and be unable to get a fair trial in the U.S.
Assange remains in London's high-security Belmarsh Prison, where he has been since he was arrested in 2019 for skipping bail during a separate legal battle. Before that, he spent seven years inside the Ecuadorian Embassy in London to avoid extradition to Sweden to face allegations of rape and sexual assault.
Sweden dropped the sex crimes investigations in November 2019 because so much time had elapsed, but British judges have kept Assange in prison pending the outcome of the extradition case.