A legal challenge arguing that the U.K. parliament, not the Prime Minister, can trigger Article 50 of the Lisbon Treaty and officially start the country's withdrawal from the European Union risks inadvertently pushing the U.K. out of the union rather than keeping it in, a pro-EU activist told The Street.

Roger Casale, founder and CEO of New Europeans -- an organization advocating for Britain to remain in the EU -- made the remarks in an interview ahead of a High Court verdict on the issue due on Thursday. (You can read more about the legal challenge in this article published by The Street last month.)

"I understand why people are doing it and all forms of resistance to Brexit are welcome, but it could have unforeseen consequences politically and in fact jeopardize the attempt to keep Britain in the European Union," he said.

Whichever side loses is expected to appeal, but the verdict, due at 10 am GMT, is closely watched by investors, who hope that if the courts decide that parliament must vote on triggering Article 50, a "soft Brexit" or even no Brexit is possible.

This is because a majority of lawmakers are in favor of remaining in the EU. This puts the U.K. parliament at odds with the results of the June 23 referendum, when 52% of people voted for Brexit.

So far, the economy has held up remarkably well. The vote has crashed the pound by 18% against the dollar and around 15% against the euro, but Brexit supporters say this is good for exports. Still, warnings have begun to appear that inflation could go as high as 4% next year, and even two of Brexit's biggest corporate supporters seem to be showing signs of buyer's remorse.

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