LONDON – Julian Assange, founder of the anti-secrecy website WikiLeaks, lost a British Supreme Court appeal to block his extradition to Sweden to face rape claims after questioning a technicality of his detainment 18 months ago. His lawyer said she may challenge the decision.

The court on Wednesday rejected Assange’s argument that the Swedish prosecutor who investigated the sexual assault claims and issued a European arrest warrant for him in 2010 wasn’t a “judicial authority” under European Union law.

Assange’s lawyer Dinah Rose said she may seek to re-open the matter by challenging the court’s handling of the case. While Assange, 40, wasn’t in court, dozens of his supporters attended the hearing and protesters holding signs surrounded the court’s entrance. Britain’s Crown Prosecution Service agreed not to return him to Sweden until June 13, and his legal team was given 14 days to file a challenge.

The claims by two women that Assange sexually assaulted them became public around the same time he posted classified U.S. military and diplomatic cables on the WikiLeaks website, creating controversy for U.S. President Obama’s administration.