Wikileaks founder Julian Assange has asked the UK's Supreme Court to reopen his appeal against his extradition to Sweden over alleged sex offences.
The court had rejected his argument that a European arrest warrant was invalid in the case.
But his lawyers said some of the judges had reached their decision on a legal point not raised during the hearing.
Mr Assange, who denies any wrongdoing, has now formally lodged a challenge. He was arrested in London in 2010.
Swedish prosecutors want to question Mr Assange over allegations of rape and sexual assault made by two female Wikileaks supporters in mid-2010 but have not filed any charges.
The Wikileaks website published material from leaked diplomatic cables, embarrassing several governments.
The move by Mr Assange's legal team was expected and follows the Supreme Court's rejection of the Australian citizen's appeal against extradition last month.
When the Supreme Court's judgement was handed down by a 5-2 majority, Mr Assange's lawyer Dinah Rose QC pointed out that some of the justices had relied on argument that she had not had the chance to address.
No timetable has been given for an eventual decision.
The justices could choose to reject Assange's challenge. If they agree to reopen the case, they could ask both sides to offer written submissions or hold a new hearing, says BBC legal correspondent Clive Coleman.
His lawyers told the Supreme Court that the European arrest warrant was invalid because it was issued by a prosecutor and not a judge or a court, as required in the UK.
If Mr Assange loses the latest appeal, he could still take his case to the European Court of Human Rights.