WikiLeaks founder Julian Assange filed an appeal Thursday to challenge a decision of the London High Court of Justice, which opened the door for his extradition to the US on charges of spying. If the appeal is granted, the case will be heard at the UK Supreme Court.

The Queen’s Bench Division of the London High Court of Justice overturned the Westminster Magistrates’ Court’s decision earlier this month preventing Assange from being extradited. In doing so, the Court rejected Assange’s assertion that he could not be removed under the Extradition Act of 2003 due to mental health concerns, and found the US government’s assurances regarding the Assange’s treatment sufficient to allow for extradition despite the Act’s requirements.

The said assurances included a commitment by the US government not to hold Assange at its Florence, Colorado facility—the highest level maximum security prison in the country—or under “special administrative measures” such as solitary confinement, which could cause his mental condition to deteriorate. US authorities also indicated their willingness to transfer Assange to a facility in his home country of Australia to serve his sentence if convicted.

Assange’s appeal challenges the reasoning of the Queen’s Bench regarding US assurances, which he claims are discretionary and ultimately meaningless. Assange’s lawyer Stella Moris, stated that a decision on the appeal is not expected until the third week of January.

Assange has been in custody since 2019, despite serving a sentence for violating bail conditions in an unrelated case, and spent seven years at Ecuador’s embassy in London to avoid being removed to Sweden to face sexual assault allegations. These allegations were later dropped. Assange faces 175 years in prison for exposing US war crimes in Iraq, Afghanistan and Guantánamo.