A British appellate court opened the door on Friday for Julian Assange to be extradited to the US by overturning a lower court ruling that found the WikiLeaks founder's mental health was too fragile to withstand the American criminal justice system.

The High Court in London ruled that US assurances were enough to guarantee Assange would be treated humanely and directed a lower court judge to send the extradition request to the home secretary for review. The home secretary, who oversees law enforcement in the United Kingdom, will make the final decision on whether to extradite Assange.

However, the appellate court ruling handed down on Friday is likely to be appealed.

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 offence is 63 months.

Assange, 50, is currently being held at London's high-security Belmarsh Prison.