England’s most senior judges will hear arguments brought by lawyers for a number of different claimants, including an investment fund manager, a hairdresser and an expatriate living in France.
May has condemned their challenge as an attempt to “subvert” the result of the June referendum when 52 percent of Britons voted to leave the EU.
But the claimants argue either that the vote was only “advisory” and must be implemented by elected lawmakers, or that only parliament can remove the rights accorded to Britons as EU citizens.
“Parliament has taken us into the European Union and only parliament can take us out,” said lawyer John Halford of Bindmans solicitors.
Most MPs campaigned for Britain to stay in the EU and, while many have now accepted the result, lengthy debates ahead of a vote could take months, upsetting the entire Brexit timetable.
May has said she wants to trigger Article 50 — the formal procedure for starting two years of talks on departure — by the end of March at the latest.
The government has argued its right to lead Brexit falls under “royal prerogative” — a type of executive privilege used in foreign policy.
The case reflects growing pressure from MPs themselves, led by the main opposition Labour party, for a vote on the terms of Britain’s exit over fears of an abrupt departure from the single market.
The case will be heard by Lord Chief Justice John Thomas, Master of the Rolls Terence Etherton and senior Court of Appeal judge Philip Sales.
After a three-day hearing, the judges will retire and likely deliver their verdict within a few weeks.
The losing party is almost certain to appeal, but the seniority of the panel means it would probably go straight to the Supreme Court for a final decision.
Jo Hunt, a law lecturer at Cardiff University, said that although the power to trigger Article 50 “probably” does fall under royal prerogative “there are strong… arguments why it shouldn’t”.
In the first few weeks after the referendum, there was much speculation by so-called “Remainers” about legal challenges that could overturn the result.
This is now thought highly unlikely — although May accused those behind Thursday’s challenge of trying all the same.
“Those people who argue that Article 50 can only be triggered after agreement in both Houses of Parliament are not standing up for democracy, they’re trying to subvert it,” she said.
“They’re not trying to get Brexit right, they’re trying to kill it by delaying it. They are insulting the intelligence of the British people.”
The case was initially brought by 37-year-old British hairdresser Deir Dos Santos, and Gina Miller, co-founder of SCM Direct investment managers, before being joined by a group of self-styled “ordinary citizens”.
“My client was very surprised at being labelled by the prime minister as trying to subvert democracy,” said Dominic Chambers of Maitland Chambers, who represents Dos Santos.
In a sign of the persistent tensions between the two opposing camps, Dos Santos is keeping a low profile after being targeted by abuse on social media.
The offices of Miller’s lawyers, Mishcon de Reya, have also been picketed.
A third group of claimants are represented by Halford and led by expat Grahame Pigney, a “committed European” who lives in France.