By Estelle Shirbon and Michael Holden
LONDON (Reuters) - Britain's Supreme Court will decide as quickly as possible whether Prime Minister Theresa May can trigger Britain's exit from the European Union by the end of March without parliament's assent, its president said on Thursday.
Last month, the High Court decided that May could not invoke Article 50 of the Lisbon Treaty, the EU's exit clause, using executive powers known as the "royal prerogative".
"We are not being asked to overturn the result of the EU referendum," Supreme Court President David Neuberger said at the end of a four-day appeal in which the government sought to overturn the High Court ruling.
"The ultimate question in this case concerns the process by which that result can lawfully be brought into effect," he said.
The case could potentially hamper May's Brexit plans, and investors believe involving lawmakers would lessen the chances of a "hard" Brexit, where Britain gives up access to the single European market in order to impose tighter immigration controls.
Pro-Brexit critics have cast the legal battle as an attempt by a pro-EU establishment to thwart the result of June's referendum, when Britons voted by 52-48 percent to leave the EU.
The judges in the High Court case were dubbed "enemies of the people" by one newspaper while Gina Miller, the investment manager who brought the challenge, has received death threats and a torrent of online abuse.
The government's argument is essentially that under Britain's unwritten constitution, it can make or leave international treaties without parliamentary assent.
The challengers argue that triggering Article 50 would inevitably mean citizens would lose rights granted by parliament and so only MPs could take these away.
NOT IF, BUT HOW
The Scottish and Welsh governments and lawyers for Northern Irish challengers all joined the case against the government. Scotland's nationalist executive, which opposes Brexit, argued that the Scottish parliament should be consulted.
"(Brexit) is almost the most divisive political event that has happened over the last several decades," Richard Gordon, law officer for the Welsh government, told the court on Thursday. "Who is going to judge what happens next? According to law ... it must be parliament."
If May wins, she can follow her planned timetable for invoking Article 50. If she loses, she might need to bring in a parliamentary bill, albeit one containing just a single line.
On Wednesday, parliament overwhelmingly voted to back a motion supporting her timetable, which government lawyer James Eadie said was highly significant and "legally relevant".
"The House of Commons has given specific approval to the government to give (Article 50) notice and indeed has called on them to do so by a particular date," he said.
Echoing comments made by her lawyer to the Supreme Court on Wednesday, lead challenger Miller said she did not believe the motion in parliament had any bearing on the case.
"Our case has nothing to do with politics – it concerns legal process and the constitution. It is not about if we leave the EU, it is about how we leave the EU," she said.