The U.K. must hold a vote in Parliament before starting the two-year countdown to Brexit, a panel of London judges decided, setting up a constitutional confrontation at the country's Supreme Court next month.

Triggering the departure from the European Union would "inevitably" change domestic law without Parliament's approval, Judge John Thomas said Thursday, delivering a decision that is a setback for Prime Minister Theresa May's plan to unilaterally start the process by the end of March by invoking Article 50 of the Lisbon Treaty.

The ruling sent immediate shockwaves through London's financial and political enclaves. The pound—the worst-performing major currency in 2016—rose to a three-week high against the dollar, and lawmakers on both sides of the debate grappled over whether the decision would force May to alter her plans. The government said it can still press ahead with the March timetable even if it loses an appeal.

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