Here is the argument the lead claimant Gina Miller used to win the historic High Court Brexit ruling
In a landmark verdict, three of the UK's most senior judges said the government is legally required to pass an act of parliament before initiating Britain's formal departure from the European Union.
Gina Miller, an investment manager based in London, was the lead claimant in the case.
Her legal representative, Lord Pannick QC, successfully argued that using royal prerogative to invoke Article 50 will mean statutory rights enjoyed by Brits as EU citizens being slashed without parliamentary involvement.
We interviewed Miller in August to find out what motivated her to launch one of the most important constitutional cases in Britain's history. This is what she told us in August this year.
Business Insider's Adam Payne: Is it safe to assume you wanted Britain to remain in the EU?
Gina Miller:Actually, I managed to fall out with both sides - Leave and Remain. I thought the question was far too binary and I kept saying I was not a remainer - I was for remain, reform, and review.
As people in business know, if you just sit on your hands and don't progress with the changing environment, you won't reform and improve the existing relationships you have. Whilst I agree that we should remain, I don't believe that it's something to be taken for granted.
There is an awful lot of work we could do. What has frustrated me more than anything is the fact that the UK could be part of that reforming agenda and even leading it and we wasted that opportunity.
Payne: So what prompted you to launch this legal case?
Miller: On the morning of the 24th, like so many people, I was absolutely stunned.
I was very aware that there was no particular plan to leave and it was fought with a huge amount of unknown. I was very alarmed by the out vote because I felt people had been lied to. But also because there was no plan B or any plan at all for what it would actually mean if we voted out.
Payne: What was it in particular that bothered you?
Miller: The word which kept ringing in my ear over and over again was sovereignty. I started looking into it further because there were 3 three things which particularly worried me.
1) It is clear Article 50 has to be triggered along a number of procedural steps and with regard to our constitution. If it isn't, it will weaken our negotiation position with the EU member states.
2) If we trigger Article 50 under royal prerogative, what is stopping the EU turning around and saying "well, you've actually triggered Article 50 illegally, you've now forfeited your two-year negotiation period, so leave?" It really isn't beyond the EU to do that. Article 50 is very poorly written and raises more questions more answers.
3) We must remember that the UK doesn't have a written constitution - it is made up of precedent. If we set the precedent that a government can use their royal prerogative to take away people's human rights, that is taking us into a very dangerous political environment.
Miller: I believe these things should be debated and looked at in parliament. It would be the first time that we would have a proper, serious, grown-up debate about all the factors that will influence us leaving the EU. There should be a debate about the consequences for different sectors. MPs should listen to their constituents. Then, if MPs vote in favour of invoking Article 50, primary legislation.
But don't forget, the court could turn around and say "we agree that we are leaving so now we need an act of parliament" and it could be as simple as that. It wouldn't necessarily mean that the process gets delayed. If MPs vote to pass the act then Article 50 would be triggered.
We must not underestimate or forget the anger in Europe about our vote. The UK has always had a special relationship with the EU. They've given to us significantly over the years and we've always had a veto. They're very angry that we've had this relationship yet we still threaten the union.
Payne: How likely is the case to be successful?
Miller: It's very hard to say. It's going to be the biggest constitutional case for hundreds of years because we've never debated these things before. You know the courts are taking it seriously because they've exercised something called a "legal leapfrog" which means because time is of the essence, we are going to skip the low courts and so straight to Lord Justice and then Supreme Court.
To me, it's very clear that we cannot have a government using royal prerogative to take away peoples' human rights.
From a legal point of view, the referendum was advisory. I know it was the venting of anger and peoples' dissatisfaction with the political environment, but that's not what I'm concerned about. I'm very much concerned with the procedural substance of what we do because I believe we are in a much worse place if we don't do that.
Payne: Speaking of anger, have you come in for any criticism for launching this case?
Miller: I'm the only named client now but originally I wasn't the only one and the other people whose names were listed as clients received unbelievably vile abuse.
[Initially, Miller was among a number of clients bringing similar cases forward. These cases were bundled into a single lawsuit with Miller as the listed plaintiff]
Since my name went public on July 19, I have received the same. It has been horrendous. There are boycotts against our business. There are campaigns on Facebook encouraging people to do things to me. That fuels me more. We cannot let bullies hijack this process. It certainly isn't a comfortable place to be.
Payne: Have you spoken to any politicians about your case?
Miller: We could have done that and we debated whether we should do that but I want to be very clear that this isn't about politics. It's about procedure, policy, and substance regarding how we should leave.
Payne: How much of a preoccupation has this case become for you?
Miller: Like most entrepreneurs will admit, I don't tend to sleep a lot.
I've already got the investment business, I run a big foundation where we look after community charities, I run something called the campaign which is calling for reform in the city's code of ethics, and this case, like everything else I do, is about what is best for consumers and how we should be serving people as a society. This is just another example of that.