British appellate court opened the door on Friday for Julian Assange to be extradited to the United States on spying charges by overturning a lower court decision that the WikiLeaks founder’s mental health was too fragile to withstand incarceration in America.

The High Court in London ruled that U.S. assurances about Julian Assange’s detention, received after the lower court decision, were enough to guarantee he would be treated humanely. Assange’s lawyers say they will ask to appeal.

A lower court judge earlier this year refused an American request to extradite Assange to the US to face spying charges over WikiLeaks’ publication of secret military documents a decade ago. District Judge Vanessa Baraitser denied extradition on health grounds, saying Assange was likely to kill himself if held under harsh US prison conditions.

The US appealed, challenging the notion that Assange’s mental health made him too vulnerable to withstand the US judicial system. Lawyer James Lewis said Assange has no history of serious and enduring mental illness and does not meet the threshold of being so ill that he cannot resist harming himself.

US authorities have told British judges that if they agree to extradite Assange, he could serve any US prison sentence he receives in his native Australia.

US prosecutors have indicted Assange on 17 espionage charges and one charge of computer misuse over WikiLeaks’ publication of thousands of leaked military and diplomatic documents. The charges carry a maximum sentence of 175 years in prison, although Lewis said the longest sentence ever imposed for this offense is 63 months.

Since WikiLeaks began publishing classified documents more than a decade ago, Assange has become a lightning rod for both criticism and veneration.

Some see him as a dangerous secret-spiller who endangered the lives of informers and others who helped the U.S. in war zones. Others say WikiLeaks has publicized official malfeasance that governments wanted to keep secret.

Assange is currently being held at London’s high-security Belmarsh Prison. The High Court ordered that he remain in custody pending the outcome of the extradition case.