The Government will take its battle to start the process of leaving the EU to the Supreme Court today, in what is seen as one of the most significant constitutional battles in decades.
Senior figures involved in the critical case including Attorney General Jeremy Wright QC and businesswoman Gina Miller – who brought the case against the Government – arrived early at the central London court for the hearing.
Mr Wright will argue that Theresa May can trigger Article 50 using so-called prerogative powers – meaning MPs do not have to vote on the matter in Parliament.
The Government is appealing against an earlier decision by the High Court which ruled in favour of Ms Miller and others who want to see Parliament decide.
Ms Miller told Sky News that the court battle “is about two very fundamental constitutional questions”.
She said: “One is that Parliament is sovereign and you cannot bypass it.
“Second, the Government can’t use prerogative powers to strip people of rights and we will lose rights if we leave the EU.”
Ms Miller said that since the High Court ruling she has suffered threats and intimidation.
She added: “There have been, as you would expect, a lot of trolls and keyboard warriors.
© Getty Theresa May”But it has moved beyond that to sexual and racial threats and death threats that have come via emails, phone calls and letters to my office.
“My staff have been bombarded, my family threatened, there is even a bounty on my head via social media.”
It is a sign of the strength of feeling around the issue that the High Court judges who handed victory to Ms Miller have themselves come under attack.
Many will now be looking to the Supreme Court to overturn that decision and such is the significance of the case that for the first time ever all of the court’s eleven justices will sit together to consider the Government’s appeal.
Constitutional law expert Professor Vernon Bogdanor believes both sides have strong legal cases.
He told Sky News: “The legal strength of the Government’s position is that treaty matters are for the executive (the government) and therefore they are an exercise of prerogative power and that, I think it is agreed, would be true of every treaty.
“But the case on the other side is that the treaty concerned with the European Union is not just an ordinary treaty because it has been incorporated into British law.”
Prof Bogdanor added: “If the arguments were cut and dried they would have been settled at a much lower level.
“The fact that they are going to the Supreme Court shows that there are strong arguments on each of the debate.”
© PA Businesswoman Gina Miller arrives for the hearingThe worry for the Government is that if Parliament gets to vote it could interfere with the timetable for triggering Article 50 or, in a very unlikely scenario, thwart it.
The case was fast-tracked to the Supreme Court to avoid any accusation that the judiciary was holding up the political process, but a former attorney general fears there may be a backlash against the court regardless of which result emerges.
Dominic Grieve QC told Sky News: “The judiciary are there to uphold our constitution.
“We don’t have a written constitution in this country but we do have perfectly clear principles and they have to be interpreted by our judges.”
He added: “When the judges go about this work, whether it is the decision in the High Court or the decision that will come out of the Supreme Court, they are doing exactly what we require them to do.
“Whilst you can certainly disagree with the decision they might come up with…to attack them for doing their job is quite wrong”.
Mr Grieve’s warning is unlikely to prevent a strong reaction whatever the outcome – such are the sensitivities around Brexit.
The Government will have to wait until January to hear if it has won its appeal.
:: Watch coverage of the court hearings live on this page from 11am