LONDON, Dec 7 (Reuters) - The referendum in which Britons voted to leave the European Union is not legally binding, the Supreme Court was told on Wednesday during a hearing on who has the power to trigger Brexit.
The court is considering an appeal by the government against a ruling last month that ministers cannot invoke Article 50 of the EU's Lisbon Treaty, the first formal step in the process of leaving the bloc, without parliament's explicit approval.
If the Supreme Court upholds that ruling, the risk for the government is that proceedings in parliament could delay Prime Minister Theresa May's plan to trigger Article 50 by the end of March, watering down her Brexit strategy.
The case has inflamed passions in Britain, with pro-Brexit critics saying those challenging the government through the courts were seeking to thwart the will of the people.
On Wednesday, police announced they had arrested a man on suspicion of making racist threats against Gina Miller, the lead claimant in the case against the government.
David Pannick, the lawyer acting for Miller, told the court the June referendum, in which Britons voted for Brexit by 52 percent to 48, had important political consequences but not legal ones.
"Parliament has deliberately chosen a model which does not involve any binding legal effect," he told the 11 Supreme Court justices during an exchange on the 2015 act of parliament that set the rules for the referendum to take place.
Pannick argued that the 2015 act did not say what should happen after the referendum, and that since triggering Article 50 would effectively nullify the 1972 act through which Britain joined the EU, only parliament could authorise such a step.
Pannick's assertion that the outcome of the referendum was merely advisory goes to the heart of the controversy generated by the case.
Many politicians in May's Conservative Party and some pro-Brexit newspapers say the courts would be overstepping their constitutional role if they ignored the express will of the people.
Miller and Pannick counter that their case is not aimed at blocking Brexit but rather at ensuring parliament is properly involved in the process. They won convincingly in the High Court in November.
Supreme Court President David Neuberger questioned Pannick on Wednesday over whether parliament needed further involvement.
"One could say it (the referendum process) is parliament ceding the ground ... to the people, to a referendum. It's done that, and then it's over to the government," Neuberger said.
"It could be said to be a bit surprising that ... an act such as the Referendum Act and an event such as the referendum has no effect as a matter of law."
A stone's throw away from the Supreme Court, parliament will vote on Wednesday on whether to back May's timetable on Article 50.
Pannick said that even parliament did vote in favour of the March deadline, that would make no difference to his case that parliament, not ministers, had the power to authorise triggering the article.
"Our submission is that a motion in parliament does not affect, cannot affect, the legal issues in this case," he said. (Writing by Michael Holden and Estelle Shirbon; editing by John Stonestreet)