Lawyers seeking to force the government to hold a Parliamentary vote on whether to leave the European Union told a panel of three senior judges that Prime Minister Theresa May’s bid to trigger Brexit on her own amounts to an unconstitutional power grab.

David Pannick, who represents an investment manager pushing for Parliamentary oversight of Brexit, said May’s decision to trigger Article 50 and negotiations to leave the EU without a vote was a reckless political move that trampled upon centuries of legal precedent.

Victory for Pannick’s clients could undermine May’s plans to invoke Article 50 by the end of March. Any delays in the process could cheer investors who were worried that May has adopted an aggressive attitude that could cost the U.K. membership to the EU’s single market for trading goods and services.

“The case raises issues of fundamental constitutional importance concerning the limits of the power of the executive,” Pannick said Thursday in London on the first day of a three-day hearing. “The government is wrong to suggest that for us to challenge the legality of the proposed notion is merely camouflage.”

The prime minister’s most senior legal adviser, Attorney General Jeremy Wright, will argue the government’s case on Oct. 17, in a sign of how seriously May is taking the challenge. Failure would force the issue into the House of Commons and the House of Lords, both of which were largely pro-EU.

‘Uptight Government’

The reason “the government is so uptight about this is that if it went to a vote in parliament and the government lost that, it would just be chaos,” Robert Thomas, professor of public law at the University of Manchester, said before Thursday’s hearing. “We’ve got a Brexit crisis at the moment. But if that circumstance arose, we’d basically see not just the meltdown of the government but potentially the whole constitutional and political system.”

The pound gained Wednesday after May accepted that Parliament should be allowed a chance to debate her strategy for taking Britain out of the European Union.

The rally saw the pound regain just a fraction of its 4.9 percent loss versus the dollar over the preceding four days. It has dropped 18 percent since Britain voted to leave the European Union and almost 6 percent this month on mounting concerns over the prime minister’s hard-line stance.

The court hasn’t scheduled a date for its ruling, but any opinion would have to be handed down in time for the parties to prepare for an appeal to the Supreme Court in December.

Wright has previously said that the lawsuit was little more than an attempt to circumvent the British public’s decision in the June 23 referendum.

Unhappy Remain Voters

But Pannick said Thursday that the challenge was not an attempt by unhappy remain voters to block Brexit.

“That argument fails to recognize both the advisory nature of the referendum in parliamentary democracy,” he said. The vote “did not involve any expression of view of when and what terms we leave and what role Parliament should play.”

Lead claimant Gina Miller, who runs an investment start-up, says she wants an “honest, grown-up” debate in Parliament about the impact of Brexit on science, education and security. Other claimants include a hairdresser, Deir Dos Santos.

Lawyers for Dos Santos are relying on a three-century-old statute enacted after the overthrow of King James II. The rule, enshrined in the Bill of Rights of 1689, says that laws should not be discarded or suspended without consent from Parliament.

“It is a constitutional requirement that parliamentary sanction be obtained for loss of rights,” said Dominic Chambers, a lawyer advising Dos Santos. “What that means is Parliament must take the decision that the U.K. should leave the EU.”

The discussions in the courtroom spilled out onto the streets outside the Royal Courts of Justice on London’s Strand Thursday morning. About a half a dozen protesters dressed in the robes and wigs of British judges lambasted anybody walking through the court’s iron gates, accusing them of interfering with the will of the British people as pro-remain activists draped in European flags attempted to shout them down.

Pannick’s position echoes a September report by the House of Lords Constitution Committee which said it would be “constitutionally inappropriate” and would set “a disturbing precedent” for the government to act on the referendum without parliamentary approval.

Triggering Article 50 without a vote “will defeat rights that have been conferred by Parliament,” Pannick said. “None of the constitutional developments that have occurred come close to affecting the basic truth that Parliament is sovereign.”