By Michael Holden
LONDON, Oct 13 (Reuters) - A legal bid to force the British government to seek parliamentary approval before starting the formal process of leaving the European Union begins on Thursday, with ministers calling it an anti-democratic tactic to delay Brexit.
Prime Minister Theresa May has said she will trigger Article 50 of the Lisbon Treaty, the mechanism by which Britain begins a two-year process to exit the European Union, 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.
Thursday's case in London's High Court comes amid wider calls for lawmakers to have a vote over the shape of Brexit negotiations, with many politicians arguing the June 23 referendum only revealed that Britons wanted to leave the bloc.
They voted by 52 percent against 48 percent to leave the EU, with the "Leave" campaign focusing on the issue of immigration.
There is growing concern among some in parliament and in the business community that May will go for a "hard Brexit", with Britain leaving the EU's single market and imposing tight immigration controls, damaging trade and investment.
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 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," investment manager Gina Miller, the lead claimant, 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."
The opposition Labour party on Wednesday demanded a debate on Brexit saying there should be detailed scrutiny of and a vote on the government's plans in the Brexit negotiations.
"The mandate on the 23rd of June was not a mandate on the terms (of an EU exit deal)," said Keir Starmer, Labour's Brexit spokesman.
May and Brexit minister David Davis reiterated that the government had the right to trigger Article 50, but promised lawmakers would be allowed their say.
"The premise on which we are advancing is that we will have proper scrutiny," Davis said. "But it is not one where we will allow anyone to veto the decision of the British people."
The legal challenge beginning on Thursday simply centres around who has the right to trigger Article 50 under Britain's unwritten constitution.
The litigants, which also include a hairdresser, British expatriates and a group which calls itself the "People's Challenge", say that Britons' rights established by a 1972 act of parliament cannot be withdrawn by the government alone and that the referendum was only advisory.
The government's top lawyer, Attorney General Jeremy Wright will argue the case for ministers.
Lord Chief Justice John Thomas, Britain's most senior judge, will oversee the arguments over three days with a verdict expected by the end of the month. It is likely that the matter will be appealed to the British Supreme Court for a final decision before the end of the year.
If the court challenge is successful, the government could be forced to pass legislation authorising Article 50 to be triggered, requiring a vote by lawmakers, the majority of whom voted to Britain to remain in the EU.
However, a Reuters poll of lawmakers found a third who voted "remain" would now support triggering the Brexit process, although it might face more opposition in the upper chamber, the House of Lords.
Some lawyers involved in the legal case also believe May might still choose to ignore the judges' decision if it goes against her.
(Editing by Guy Faulconbridge and Angus MacSwan)