Britain's Supreme Court says the Government must get parliamentary approval before starting the process of leaving the European Union, potentially delaying Prime Minister Theresa May's plan to trigger negotiations by the end of March.
Supreme Court President David Neuberger says the vote was a majority of 8-3.
The court also unanimously decided that there's no need to consult Scotland, Wales, Northern Ireland on Brexit.
British Attorney General Jeremy Wright said that the Government will comply with the ruling, and that a statement will be made in Parliament later.
Tuesday's ruling raises hopes among pro-EU politicians that they will be able soften the terms of the UK's withdrawal from the bloc.
"Leave" campaigners have objected, saying Parliament shouldn't have the power to overrule the electorate, which voted to leave the EU in a June 23 referendum.
May wanted to use centuries-old powers known as royal prerogative to invoke Article 50 of the EU treaty and launch two years of exit talks.
The powers - traditionally held by the Monarch -permit decisions about treaties and other issues to be made without a vote of Parliament.
Here are answers to some key questions about the case.
WHY IS IT SO IMPORTANT?
Prime Minister Theresa May has pledged to start the Article 50 process, which is expected to last two years, by the end of March.
European leaders want to get talks underway, and some British voters who backed Brexit in a June referendum are getting impatient.
Having Parliament play a direct role could slow the process down.
Although the leader of the opposition Labour Party says its legislators will back Brexit out of respect for the referendum result, the process could easily be delayed in the House of Commons or the House of Lords.
WHAT HAPPENS IF THE GOVERNMENT LOSES ITS APPEAL?
May's ministers have prepared several draft pieces of legislation that could be introduced in Parliament.
The goal is to craft a very short, limited bill that would give May the authority to invoke Article 50, but would be difficult to amend or tamper with.
The bill that is introduced would be shaped in large part by the actual decision, which may spell out specific requirements.
The government has drawn up various contingency plans to deal with guidelines that may be imposed by the court. The judges could say, for example, that a single vote in Parliament would be sufficient, or it could require the more time-consuming drafting of a law.
The government would likely move quickly to introduce a bill with the goal of meeting the March deadline.
COULD A LOSS FOR THE GOVERNMENT MEAN AN END TO BREXIT?
It's very unlikely Parliament would vote against the government's proposal given the referendum result.
It would be seen by Brexit supporters and many who opposed it as an anti-democratic measure thwarting the will of the people.
WHAT IF THE GOVERNMENT WON?
A reversal of the lower court ruling would give May the authority to start Article 50 negotiations when she chooses without input from Parliament.
She has said, however, that Parliament will be asked to approve any Brexit deal reached at the conclusion of the talks.