Gina Miller, a London-based investment manager for the firm SCM Private, is spearheading a landmark legal case which could have huge ramifications for Brexit and the UK's entire constitutional landscape.

Miller, as Business Insider's Jim Edwards reported earlier this month, is the claimant in a historic lawsuit which will be heard by the Supreme Court in October.

She argues the prime minister should not be able to use royal prerogative to invoke Article 50 -- the official process of leaving the European Union. Instead, Theresa May, or whoever is PM, should be legally required to secure parliamentary approval before triggering Britain's departure from the 28-nation bloc.

The stakes of the case could not be higher. If the court rules in Miller's favour, May will not be able to begin Britain's official withdrawal from the EU until the majority of MPs vote in favour of it. In theory, it would give MPs the license to severely delay Brexit.

BI caught up with Miller on Tuesday morning to discuss the case, how likely it is to be successful, and the impact it could have on both her own profile and Britain's relationship with the EU.

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.

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.

Payne: So what exactly do you want to see happen?

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.

To me, it's very clear that we cannot have a government using royal prerogative to take away peoples' human rights

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.

We cannot let bullies hijack this process

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.

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