The U.K. government's Brexit plans were dealt a hammer-blow Thursday after the country's High Court ruled that lawmakers would need to vote on triggering the Article 50 exit clause from the European Union.

But the woman behind the historic case against the government, Gina Miller, is no stranger to conflict, having previously called for more transparency in the fund management that raised the ire of her industry colleagues.

The 51-year-old, who was born in Guyana but grew up in Britain, is the co-founder of SCM Direct, a fund which offers low-cost, passive strategies but with active asset allocation management. She formed the firm in 2009 with her her husband Alan, another industry veteran with stints at Gartmore, Jupiter and New Star on his resume.

In 2012, the pair set up the True and Fair campaign that called for more transparency in asset management fees and an end to hidden charges. Her advocacy, however, has earned her the unfortunate - many would say inappropriate - nickname in the industry as the 'Black Widow'.

"That nickname is wrong on so many levels," she told the Financial Times in a recent interview. "But I do not and will not let other people bring me down. I believe that level of abuse means I am doing something right for investors."

She is now at the receiving end of the wrath of Brexiters across the U.K.

The closely-watched case was brought by a group of British residents - including Deir Dos Santos, a hairdresser born in Spain and Grahame Pigney, a France-based expatriate who used crowdfunding to pay for lawyers. The plaintiffs argued that a unilateral decision by Prime Minister Theresa May to trigger Article 50 would effectively override a 1972 statute that enshrines European law in U.K. statutes.

"We decide that the Government does not have power under the Crown's prerogative to give notice pursuant to Article 50 for the U.K. to withdraw from the European Union," the three Justices assigned to the case wrote in their formal ruling.

The case was argued by the law firm Mishcon de Reya, which will now be preparing to defend its position against an appeal by the government. That case will be heard by the Supreme Court in early December.

Prime Minister May said after the ruling that the government intends to stick to the deadline of triggering the exit clause by the end of March 2017. "We believe the legal timetable leaves time for that," a spokeswoman for the Prime Minister said in a statement.

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