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British MPs will have their say on whether the UK can trigger Article 50 of the Lisbon Treaty, which will start the process of leaving the European Union, a High Court has ruled.

SEE ALSO:Brexiter regrets leave vote, chokes up in live radio interview

In one of the most important constitutional cases in the country's history, three judges ruled that Prime Minister Theresa May cannot use her royal prerogative to begin formal discussions with the EU on her own.

The lord chief justice, Lord Thomas of Cwmgiedd, said that “the most fundamental rule of the UK constitution is that parliament is sovereign.”

The judgment is a huge setback for Theresa May, who had argued that the referendum results mean MPs do not need to vote. Campaigners called this unconstitutional.

The government said it is disappointed by the ruling and will appeal the judgement.

It says Britons voted to leave the bloc "in a referendum approved by Act of Parliament. And the government is determined to respect the result of the referendum."

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Following the ruling, sterling rocketed in value:

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Prominent Brexit campaigner and outgoing Ukip leader Nigel Farage says he fears that those who voted for Brexit are going to be betrayed:

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Lib Dem leader Tim Farron, instead, said taking back power means giving parliament a vote:

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Labour leader Jeremy Corbyn said the ruling "underlines the need for the government to bring its negotiating terms to parliament without delay".

"Labour respects the decision of the British people to leave the European Union. But there must be transparency and accountability to parliament on the terms of Brexit," he said.

"Labour will be pressing the case for a Brexit that works for Britain, putting jobs, living standards and the economy first."

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Britain voted to leave the EU in June by a margin of 51.9% to 48.1%.

Negotiations about the terms of the UK's exit cannot begin until Article 50 is triggered.

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