Prime Minister Theresa May must seek approval from the British Parliament before beginning the process to leave the European Union, the nation's High Court ruled Thursday morning.
The decision marks a setback for Leave campaigners who decisively voted in June to withdraw from the 28-nation bloc. May's government said it would appeal the ruling, with a hearing set to take place next month, the Guardian reported.
The Supreme Court's lord chief justice declared the executive government alone could not invoke Article 50 of the Lisbon Treaty, the tool that would begin formal exit negotiations with the EU, without a parliamentary vote.
"The most fundamental rule of the U.K. constitution is that parliament is sovereign," the lord chief justice said.
May's official spokesman told the BBC the government had "no intention of letting" the ruling "derail Article 50 or the timetable we have set out. We are determined to continue with our plan."
May has vowed that the U.K. would begin Brexit negotiations in March, emphasizing that U.K. control over the nation's borders and immigration would be a primary priority during withdrawal talks.
U.K. Independence Party leader Nigel Farage said he feared a "betrayal" of the 52 percent of voters who backed leaving the EU during the referendum. Still, while most lawmakers opposed the decision to leave the bloc, it would be politically harmful to upend the referendum vote.