The UK's highest court will hear Julian Assange's extradition case.

• In December, the US government secured a victory in London's High Court, bringing Assange one step closer to being extradited from the UK to the US.
• In 2010 and 2011, Assange published hundreds of thousands of classified military documents and diplomatic cables, which led to his being wanted by U.S. authorities.

The U.K. High Court ruled on Monday that Julian Assange can challenge the decision to extradite him to the U.S. on espionage charges.

In December, the U.S. government won an appeal in London's High Court, bringing Assange closer to being extradited from Britain to the United States. However, on Monday, the court ruled that he could now take an appeal to the country's Supreme Court.

In 2010 and 2011, Assange published hundreds of thousands of classified military documents and diplomatic cables, which put lives in danger and led to 18 charges against him. As a result, he faces a 175-year prison sentence.

In early 2021, a U.K. judge decided that Assange should not be extradited to the U.S., as his lawyers argued that he posed a high risk of suicide in an American prison.

Since 2019, Assange, an Australian national, has been incarcerated in the U.K.'s Belmarsh prison. Prior to that, he resided in Ecuador's London embassy for several years to evade extradition to Sweden on charges of sexual misconduct.

In late 2019, the rape allegation against Assange was dropped by Swedish prosecutors, ending the nine-year investigation and removing the need for the U.K. to decide on extradition.

A diverse group of individuals, including actress Pamela Anderson, fashion designer Vivienne Westwood, British lawmakers, and human rights organizations, publicly support his freedom and call for it.

The U.S. government's victory in its appeal last month was condemned by Amnesty International as a "travesty of justice," with the organization arguing that Julian Assange's indictment poses a "grave threat to press freedom both in the United States and abroad."

The organization stated that if the ruling were upheld, it would weaken the crucial function of journalists and publishers in investigating governments and exposing their wrongdoings, making journalists feel threatened everywhere.

In 2006, Assange established WikiLeaks, which he later characterized as a vast repository of the world's most suppressed information.

—CNBC’s Sam Shead contributed to this article.