Wikileaks founder Julian Assange faces a further wait to find out whether he can bring an appeal against his extradition to the United States, the British High Court ruled on Tuesday.

According to the ruling, a decision on the appeal has been adjourned to May 20 as the court is seeking assurances from the United States, including that Assange could rely on the First Amendment to the U.S. Constitution and that he would not be sentenced to death if convicted.

"If those assurances are not given, then leave to appeal will be given and there will then be an appeal hearing," a summary of the judgment said.

Speaking outside the High Court in London, Assange's wife Stella Assange said she is "astounded" by the court's decision to delay her husband's appeal.

"What the courts have done has been to invite a political intervention from the United States," she said. "I find this astounding."