London (CNN) Britain's Supreme Court has ruled that the UK government must hold a vote in parliament before beginning the process of leaving the European Union.
The decision is a complication for Prime Minister Theresa May, who wanted to trigger Article 50 of the Lisbon Treaty -- the legal mechanism that begins the process of leaving the EU -- by the end of March. Doing so would open the door for EU negotiations, which are likely to last two years.
May had promised lawmakers a vote on the outcome of the talks, but wanted to begin the process without a decision in parliament.
The Supreme Court judges voted eight to three against the government, upholding a November High Court decision. The judges, who deliberated the case over four days in December, said that the legal consequences of leaving the EU were great enough to require an act of parliament to start the process.