LONDON: The president of Britain´s Supreme Court said on Monday a landmark case to decide whether the government can trigger the formal divorce process from the European Union without parliament´s approval was a matter of law and not politics.

Prime Minister Theresa May´s government has appealed to the country´s highest judicial body after the High Court ruled last month that ministers could not trigger Article 50 of the Lisbon Treaty and begin a two-year Brexit process without lawmakers´ assent.

If the Supreme Court upholds the earlier ruling, that could derail May´s planned timetable for invoking Article 50 by the end of March and upset the government´s wider Brexit plans.

The legal fight comes against a backdrop of claims by some politicians and newspapers that establishment judges want to thwart Brexit, which Britons voted for by 52-48 percent in the June referendum.

In his opening remarks Supreme Court President David Neuberger said the case was simply about the law.

"We are aware of the strong feelings associated with the many wider political questions surrounding the United Kingdom´s departure from the European Union," Neuberger said.

"However those wider political questions are not the subject of this appeal."

This appeal is concerned with legal issues and, as judges, our duty is to consider those issues impartially, and to decide the case according to the law.

This is what we shall do.

Outside the Supreme Court, located on the same central London square as parliament, pro-EU supporters wearing judges´ robes and wigs rode a double decker bus emblazoned with slogans, while another van carried a billboard that read "The Brexiteers have failed us all".

It was a reference to complaints that the government has not released a formal plan for what Brexit will look like. A small number of rival Brexit backers waved placards saying "This is an establishment stitch up".

Some lawmakers in May´s Conservative Party had called for Neuberger himself to stand down because his wife had previously posted anti-Brexit messages on Twitter.

He said all parties in the case had been asked if they wanted any of the judges to stand down and no objections had been raised.

The government´s top lawyer, Attorney General Jeremy Wright told the court the case had been brought "perfectly properly" and raised "issues going to the very heart of our constitutional settlement".

The case is the most high-profile one the court has considered since it came into being seven years ago.

It is due to last for four days and for the first time all its 11 justices will sit on the panel with the verdict expected later in January.