Britain’s Supreme Court ruled Tuesday that Prime Minister Theresa May must get legislative approval to start the process of leaving the European Union, raising the possibility that lawmakers will delay her plans to trigger negotiations by the end of March. The 8-3 decision forces the government to put a bill before Parliament, giving members of the House of Commons and the unelected House of Lords the chance to debate and potentially offer amendments that could soften the terms of Britain’s exit from the EU, known as Brexit.