Brexit: Theresa May's challenge to overturn High Court decision begins.

Britons are about to tune into a televised courtroom drama that could end with the British Prime Minister facing a constitutional crisis.

On Monday morning in London (10:00pm AEDT), the UK’s Supreme Court will start hearing the Government’s arguments against a High Court decision

This appeal is already historic, with all 11 justices presiding for the first time in what will be the most high-profile and complex case they have faced.

The British Attorney-General will argue on behalf of the Government that the High Court judges relegated the Brexit vote to a footnote and that the referendum should not be dismissed as “merely a political event”.

He will argue the issue “cannot be resolved in a vacuum, without regard to the outcome of the referendum”.

A majority of British voters — 17.4 million — supported the UK leaving the European Union in the June 23 referendum.

How did it get here?

It ended up in court after several complainants, including businesswoman Gina Miller, said the Prime Minister did not have the right to take Britain out of the EU without parliamentary approval.

Ms May had promised she would trigger Article 50 before the end of March next year.

That would start two years of formal talks ending in divorce from the EU by early 2019.

The three High Court judges who made the original ruling were demonised in the press.

The Daily Mail ran their photos on the front page with the headline “Enemies of the People”.

The Government announced it would appeal their decision and Monday marks the start of the four-day hearing.

Are the odds against Theresa May?

Former UK Independence Party leader Nigel Farage had threatened to bring 100,000 protesters to the Supreme Court, but those plans have been cancelled.

Leave.EU said it feared the march would be hijacked by extremist groups.

All indications are that the Government will lose the case.

Professor Michael Zander, from the London School of Economics, predicted the justices would rule 11-0.

“I would be surprised if the Attorney-General and his team of supporting QCs and other lawyers have given ministers reason to hope that there was any great hope of the High Court’s unanimous and very strong decision being reversed,” he wrote for the legal magazine Counsel.

Some have wondered, if the odds are against her, why Ms May is spending money and time going through with an appeal.

The Prime Minister would likely win a vote in Parliament and get permission to trigger Brexit, but there would be risks.

She could be forced to reveal her negotiating tactics before she was ready.

Pro-Remain MPs could succeed in getting amendments to the bill and a lengthy debate would amplify the existing fissures in the Conservative party.

And it could delay her self-imposed end-of-March deadline to trigger Brexit.

Ms May wants to be seen as a leader who follows through with her promises and is not derailed by squabbling.

An added complication for Ms May is that the Welsh and Scottish governments (both strongly anti-Brexit) have been given leave to appear before the court as well to argue that they have a right to have a say over the process.

Decision due Thursday

The justices finish hearing submissions on Thursday in London and will reserve their decision and deliberate.

The judgement is expected to be delivered in early January by Lord Neuberger, the court president, and will be broadcast live.

Details of the case and each side’s argument can be found here,

This post originally appeared on ABC News.