Ministers do not need to ask permission from Northern Ireland, Scotland or Wales before triggering Article 50, the Supreme Court has ruled.
The UK's most senior judges said the Government was not legally compelled to consult devolved legislatures.
The so-called Sewel Convention means that the UK Parliament generally does not legislate on devolved matters without consulting the other legislatures, but the court ruled unanimously that it "does not give rise to a legally enforceable obligation".
Politicians in Northern Ireland had been preparing measures to block Brexit, if the Supreme Court had ruled Stormont had to be given a say over Article 50.
Northern Ireland voted to Remain in the EU by a margin of 56 per cent and the overwhelming majority of local parties are pro-EU.
As the only part of the UK which also shares a land border with another EU nation, in the form of the Republic of Ireland, concerns have been raised about how the border will be affected, and how this could undermine the peace process.
Nicola Sturgeon, Scotland's First Minister, had promised MSPs they will have the opportunity to vote on triggering Article 50 regardless of the outcome of the Supreme Court case.
The Scottish Government has put forward proposals for a differentiated settlement that would allow Scotland to stay in the single market while the rest of the UK leaves.
Writing in the Daily Record, Ms Sturgeon said: "This isn't some academic debate—removing us from the largest single market in the world would be devastating for people's jobs and living standards."
The Government lost its appeal this morning after arguing it did not need to seek a Parliamentary vote on starting the Brexit process. A bill is thought to be heading to Parliament later this week.
Giving a short summary of the court's findings, Lord Neuberger stressed: "The issues in these proceedings have nothing to do with whether the UK should exit from the EU, or the terms or timetable for that exit.
"The main issue is whether the Government can trigger Article 50 without the prior authority of an Act of Parliament."
A spokesperson for Labour leader Jeremy Corbyn 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."
Some of Mr Corbyn's MPs are pushing for a referendum on whatever Brexit deal the Government reaches.
Gina Miller, the lead claimant, said outside the Supreme Court: "There is no doubt Brexit is the most divisive action for a generation ... but this wasn't about politics..
"In Britain we are lucky to voice legitimate concerns and views. I have therefore been shocked by the personal abuse I have received from the many quarters for simply asking a legitimate question."
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