LONDON — Prime Minister Theresa May must secure the approval of Parliament before she can begin the process of taking Britain out of the European Union, the Supreme Court ruled on Tuesday. The ruling, which upholds an earlier decision by the High Court in London, creates another hurdle for Mrs. May, who has promised to begin a two-year, irreversible process of exit negotiations by the end of March by invoking the European Union’s Article 50, the legal mechanism for leaving the bloc. In its ruling, the court noted that Parliament had approved the 1972 legislation that enabled the country to...