WikiLeaks founder Julian Assange made what could be his last appearance in any British court on Thursday as Swedish authorities warned that opposing his extradition could turn European law on its head.

The Supreme Court in London heard two days of complex arguments from lawyers for both sides as the 40-year-old Australian seeks to avoid being sent to Sweden for questioning over allegations of rape and sexual assault.

The court is Mr Assange’s final avenue of appeal in British law, and his case rests on a single point – that the Swedish prosecutor who issued a warrant for his arrest in December 2010 was not a valid judicial authority.

Clare Montgomery, the lawyer for the Swedish authorities, told the panel of seven judges on Thursday that it would “undermine” the fast-track European Arrest Warrant system if Mr Assange’s appeal was allowed.

Citing legal examples from France, Denmark, the Netherlands, Ireland and even Cambodia, she said victory for Mr Assange could lead to at least eight European countries being barred from using the warrant system.

But in her closing argument, Mr Assange’s lawyer Dinah Rose said Ms Montgomery’s case was “untenable”, and that a prosecutor was not an independent, impartial judicial authority. The white-haired Mr Assange did not comment as he left the grand court building, which is opposite the Houses of Parliament in central London.

He pushed his way through a hail of flashbulbs in the darkness before getting into a taxi, while dozens of supporters of his work on his anti-secrecy website shouted “Well done, Julian!”

The Supreme Court judgement is expected within a matter of weeks. Court officials said it would likely be handed down in person by the judges to the lawyers in the case.t.

If the Supreme Court rejects his appeal, Mr Assange will have exhausted all his options in Britain but he could still make a last-ditch appeal to the European Court of Human Rights in Strasbourg, France.