By Michael Holden and Estelle Shirbon
LONDON (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 need parliament's consent to invoke Article 50 of the EU's Lisbon Treaty, the first formal step in the process of leaving the bloc.
The government's fear is that parliament could delay Prime Minister Theresa May's plan to trigger Article 50 by the end of March and water down ministers' 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 challenger.
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David Pannick, the lawyer acting for Miller, told the court the Brexit referendum had important political consequences but no legal force.
"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 Referendum Act that set the rules for the referendum.
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 authorize such a step.
Supreme Court President David Neuberger reflected out loud on the assertion that the referendum was merely advisory.
"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," he said.
The issue 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 by putting obstacles in the government's chosen path.
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.
The government of Scotland, where a majority voted to remain in the EU in the referendum, intervened in the Supreme Court case on Wednesday, siding with Miller and the other challengers.
The Edinburgh executive, run by the Scottish National Party, is looking for ways to prevent Scotland from being taken out of the EU against its will, and has raised the possibility of a future referendum on independence from the United Kingdom.
Its chief legal officer, James Wolffe, argued that while Edinburgh could not veto Brexit, the process should unfold in parliament to respect Scotland's rights under the constitution.
(Writing by Michael Holden and Estelle Shirbon; Editing by John Stonestreet and Hugh Lawson)