Lord Chief Justice for England and Wales John Thomas (C) gestures in the Prince's Chamber before the State Opening of Parliament in the House of Lords, at the Palace of Westminster in London Britain May 27, 2015. REUTERS/Suzanne Plunkett. Suzanne Plunkett/Reuters One of the most important constitutional cases in Britain's history came to an end on Tuesday and the case for forcing the government to ask parliament's permission to trigger Article 50 is a lot stronger than first imagined.

Since Thursday, three of the UK's most senior judges heard arguments regarding whether prime minister Theresa May is legally obliged to pass an act of parliament before invoking Article 50.

The case, which Business Insider attended, was held at the Royal Courts of Justice in central London.

A long line of people snaked around the court building hours before the doors opened each morning, a level of interest which had not been seen for any case at the court before, a security guard told us. And protesters gathered outside the court while the case was being heard.

—Charlie Mullins OBE (@PimlicoPlumbers) October 17, 2016

That is because the case is a blockbuster. The final verdict will have a huge say in how and when Britain leaves the European Union. It will set a historic constitutional precedent.

After three days of intense discussion and interrogation, it was clear that the case being made against the UK government was much more compelling than most commentators and media had initially believed.

Lord Pannick QC, who was representing lead claimant Gina Miller, who BI interviewed in August, argued strongly that triggering Article 50 will mean statutory rights enjoyed by Brits as EU citizens — like the right to vote to in EU elections and refer a legal dispute to the European Court of Justice — will be destroyed in an instant.

The government, Pannick added, cannot use prerogative power to do this. He said UK law demands MPs to give Theresa May their permission before the prime minister triggers Article 50.

The consequence of notification is to destroy rights and take the preservation of rights out of parliament's hands. This cannot be done

"There is no dispute, that once notification is given, there is a direct causal link between notification and the removal of statutory rights," Pannick said.

"The consequence of notification is to destroy rights and take the preservation of rights out of parliament's hands. This cannot be done. By the time parliament comes to look at the matter, the dye will already be set."

The government's case, represented by Attorney General Jeremy Wright, James Eadie QC, and James Coppel QC, argued that the mandate to initiate Britain's exit from the EU resides in the result of the June 23 referendum.

They added that parliament was told before the referendum that the government would act on the will of the people and that the referendum legislation did not state further legislation would be necessary to put the result into effect.

Without a doubt, though, the stand-out moment of the case was when the Lord Chief Justice twice said he was "baffled" by Coppel's claim that EU citizenship rights have never been a matter for UK parliament.

"I'm baffled," Lord Thomas said. "These rights are under treaty. If amending the treaty, parliamentary approval is needed. So, I don't understand why the content of these rights are not controlled by parliament?"

It was a shaky moment for Coppel and the government's legal team. It was just one of the many occasions where the three judges appeared to be less-than-convinced by the points they were making. It was a signal that the government really does have something to worry about.

That's not to say the verdict, which is expected in mid-November, is set in stone. As legal commentator David Allen Green noted, the case is "finely balanced" and both sides articulated strong arguments.

But, as Green later tweeted: "The government and Leavers should brace themselves for real possibility that court rules Article 50 needs parliamentary consent." This case is much more balanced than most people thought.

But the judges' verdict is unlikely to be the end of the matter. The losing side is likely to launch an appeal to the Supreme Court, a process which will inevitably force May to delay the timetable for invoking Article 50 no matter what happens.