The Supreme Court in London has given its ruling on Article 50, following a four-day hearing last December.
The highest court in the land rejected an appeal by ministers against a High Court judgment blocking their decision to begin Britain's exit from the European Union without Parliament having a say.
The Justices' decision will affect the manner in which the Government can begin the formal process of leaving the European Union.
It was won by a wide-ranging group of anti-Article 50 campaigners led by investment manager Gina Miller, 51, and hairdresser Deir Dos Santos.
What exactly has the Supreme Court decided?
Supreme Court justices ruled, by a majority of eight to three, that Prime Minister Theresa May cannot lawfully bypass MPs and peers by using the royal prerogative to trigger Article 50 of the Lisbon Treaty and start the two-year process of negotiating the UK's divorce from its EU partners.
The ruling is a blow to Mrs May, who has repeatedly said she intends to trigger Article 50 by the end of March following the clear majority in favour of Brexit in the June 2016 referendum.
But what is Article 50?
It is the formal mechanism which a nation state must use if it wishes to leave the European Union. It is part of the 2007 Lisbon Treaty, which provided a new constitutional basis for the EU by replacing two treaties which had gone before it.
It says: "A Member State which decides to withdraw shall notify the European Council of its intention. In the light of the guidelines provided by the European Council, the Union shall negotiate and conclude an agreement with that State, setting out the arrangements for its withdrawal, taking account of the framework for its future relationship with the Union.
"That agreement shall be negotiated in accordance with Article 218(3) of the Treaty on the Functioning of the European Union. It shall be concluded on behalf of the Union by the Council, acting by a qualified majority, after obtaining the consent of the European Parliament.
"The Treaties shall cease to apply to the State in question from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification referred to in paragraph 2, unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period."
Why was the Supreme Court involved?
A group of campaigners, led by businesswoman and philanthropist Gina Miller, asked the High Court to rule last year on whether the Government's proposal to invoke Article 50 using the royal prerogative was legal.
The High Court ruled in November that using the royal prerogative did not follow the correct parliamentary process, meaning that only a vote in Parliament could give the Prime Minister authority to trigger Brexit.
Because the referendum on Britain's EU membership last June was not legally binding, it was treated by the court as a consultative exercise, and the judges decided that an Act of Parliament was required to make Brexit legal.
The Government appealed against that ruling, which has now been decided by the highest court in the country and has set a legal precedent for generations to come.
What happens next?
After the Supreme Court upheld the High Court judgment, the matter of invoking Article 50 becomes far more complicated.
A one-line Bill is expected to be drawn up and debated in Parliament, and Mrs May will be hoping that if that is the case, the Bill can go through the Commons and the Lords swiftly before becoming law as an Act of Parliament.
But both Houses of Parliament have plenty of Remainers in them, and they may suggest amendments and additions to the Bill that could slow the whole process up.
Labour vows not to 'frustrate' Article 50 process
Labour will not "frustrate" the process for invoking Article 50, Jeremy Corbyn said after the Supreme Court ruling.
But the Labour leader said his party would seek to amend the legislation the Government has been forced to produce in order to prevent Theresa May allowing the UK to become a "bargain basement tax haven".
He said: "Labour respects the result of the referendum and the will of the British people and will not frustrate the process for invoking Article 50.
"However, Labour will seek to amend the Article 50 Bill to prevent the Conservatives using Brexit to turn Britain into a bargain basement tax haven off the coast of Europe.
"Labour will seek to build in the principles of full, tariff-free access to the single market and maintenance of workers' rights and social and environmental protections.
"Labour is demanding a plan from the Government to ensure it is accountable to Parliament throughout the negotiations and a meaningful vote to ensure the final deal is given Parliamentary approval."
What has the Government said about the ruling?
The Government will do "all that is necessary" to implement the Supreme Court ruling that Parliament must give its approval to trigger the process of leaving the EU, Attorney General Jeremy Wright has said.
Mr Wright said the Government was "disappointed" by the final decision in its historic battle over who has the right to authorise the start of withdrawal negotiations under Article 50 of the EU treaties.
But he said that ministers will comply with the ruling, which effectively means that MPs must be given a vote on Article 50.
Speaking on the steps of the Supreme Court, the Attorney General said: "The Government will comply with the judgment of the court and do all that is necessary to implement it."