By Michael Holden

LONDON (Reuters) - Britain's vote in a June referendum to leave the European Union had no constitutional substance, according to lawyers leading a bid to force the government to seek parliamentary approval before formally starting the Brexit process.

"It was an advisory referendum, no more than that," David Pannick told the High Court on Thursday.

Prime Minister Theresa May has said she will trigger Article 50 of the EU Lisbon Treaty, the mechanism by which Britain begins a two-year process to leave the bloc, by the end of March next year and there will be no parliamentary vote beforehand.

But she is facing a legal challenge over whether the government can use a historical power known as royal prerogative to decide when, how and whether to make this decision.

Several ministers have called this week's legal challenge an attempt to subvert the democratic process.

Pannick, representing the lead claimant, investment manager Gina Miller, denied the case was "merely camouflage" by those who wanted to stay in the EU and said it raised questions of fundamental constitutional importance.

He said the government should not use an ancient power to strip away rights such as freedom of movement for people, trade and services that were granted by the 1972 act of parliament that sealed Britain's entry into the bloc.

"Rights that have been created by parliament cannot be taken away by a minister," Pannick told the court.

"The court is not concerned with the political outcome of withdrawing this country from the European Union. Our legal claim is in support of parliamentary sovereignty."


The case has aroused passions beyond the courtroom, with Britain's population still profoundly polarized over Brexit.

Lead claimant Miller said she had had death threats since the case started. "It's been horrendous," she told Reuters.

The government's top lawyer, Attorney General Jeremy Wright, will respond to Pannick's arguments later in the hearing, but a final decision is not expected for some weeks as whichever side loses is likely to appeal to the Supreme Court.

Sterling has fallen to 31-year lows since May announced the date she intended to trigger Article 50, and the court case is being closely watched by market players who believe that if there is a vote in parliament, the greater the chance there is of a "soft Brexit" or delays to the process.

May has accused those behind the action, some of whom openly admit they did not want Britain to leave the EU, of trying to subvert the referendum result.

The claimants reject this, saying they do not want to hinder or hijack the process but merely bring legal certainty and proper democratic scrutiny.

"I don't see how a court case can block Brexit," Miller, told Reuters last week. "I am saying we have parliament, scrutiny and then a vote rather than an antiquated power which is secretive and bypasses parliament."

(Editing by Estelle Shirbon)