A British judge on Monday ruled that WikiLeaks founder Julian Assange should not be extradited to the United States to face espionage charges for publishing secret documents online, finding he was at risk of suicide.

District Judge Vanessa Baraitser said extradition of the Australian publisher would be "oppressive by reason of mental harm and I order his discharge".

She said if detained in the United States, Assange "faces the bleak prospect of severely restrictive detention conditions designed to remove physical contact and reduce social interaction and contact with the outside world to a bare minimum".

"He faces these prospects as someone with a diagnosis of clinical depression and persistent thoughts of suicide," she said in her ruling.

"I am satisfied that the risk that Mr Assange will commit suicide is a substantial one."

Assange was remanded in custody until a bail application, which could take place later on Monday.

The 49-year-old wiped his forehead as the decision was announced while his fiancee Stella Moris burst into tears. She was embraced by WikiLeaks editor-in-chief Kristinn Hrafnsson.

Outside the Old Bailey court in central London, Assange supporters who had gathered since the early morning cheered and shouted "Free Assange!"

However, US and British prosecutors can appeal Baraitser's ruling.

Assange and his legal team have long argued that the protracted case, which has become a cause celebre for media freedom, was politically motivated.

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