A British appellate court opened the door Friday for Julian Assange to be extradited to the United States by overturning a lower court's decision that the WikiLeaks founder's mental health was too fragile to withstand the American criminal justice system. The High Court in London ruled that U.S. assurances were enough to guarantee Assange would be treated humanely and directed a lower court judge to send the extradition request to Britain's interior minister for review. Home Secretary Priti Patel, who oversees law enforcement in the U.K., will make the final decision on whether to extradite Assange. “There is no reason why this court should not accept the assurances as meaning what they say,'' the High Court ruling stated. “There is no basis for assuming that the USA has not given the assurances in good faith.” Read: US set to appeal UK refusal to extradite WikiLeaks' Assange Assange’s fiancé, Stella Moris, called the decision a “grave miscarriage of justice” and said Assange's lawyers would seek to appeal to the U.K. Supreme Court. “We will fight," Moris said outside court, where supporters gathered with banners demanding Assange's release. “Every generation has an epic fight to fight and this is ours, because Julian represents the fundamentals of what it means to live in a free society,” she said. Assange, 50, 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. In January, a lower court judge refused the U.S. request to extradite Assange to face spying charges over WikiLeaks’ publication of secret military documents a decade ago. District Judge Vanessa Baraitser denied extradition on health grounds, saying the Australian citizen was likely to kill himself if held under harsh U.S. prison conditions.