The government has lost it’s landmark legal battle over when it can begin the formal process of leaving the European Union — Article 50 — without parliament’s authority.

The Supreme Court has ruled that prime minister Theresa May needs to give MPs a vote on Article 50 before it can be formally triggered, following a four-day hearing in December.

The 11 judges ruled 8-3 in favour of the High Court’s decision last year.

Lord Neuberger, president of the Supreme Court, delivered a brief summary of the Court’s decision. He added that the UK doesn’t need to consult Scotland separately ahead of the process.

Lord Neuberger said:

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.

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The referendum is of great political significance, but the Act of Parliament which established it did not say what should happen as a result. So any change in the law to give effect to the referendum must be made in the only way permitted by the UK constitution, namely by an Act of Parliament. To proceed otherwise would be a breach of settled constitutional principles stretching back many centuries.

Last year, the High Court ruled Brexit shouldn’t be triggered without MPs’ approval in a major blow to the government’s plans.

May will now face a dramatic Commons showdown on her plan for a so-called hard Brexit — which means leaving the EU Single Market — and a tough battle to keep her schedule — invoking Article 50 by the end of March — on track.

The decision adds pressure to May after she was yesterday found to have kept the details of a Trident missile test failure from MPs ahead of a key vote on the programme.

Downing Street is prepared to rush through emergency legislation, and it has been reported that No10 was resigned to losing the case.

The parliament bill could be as short as one line of text and could be put to the Commons as soon as tomorrow.

What has the market reaction been?

The pound climbed immediately after the verdict, with markets cheering the added barrier to the government’s plans to quit the Single Market.

However it has since fallen back as traders react to the ruling that the UK’s devolved assemblies will not have a veto on leaving the bloc.

Scotland — which voted to remain in June’s referendum — has said it would do all it can to keep the UK with in the EU.

David Cheetham, market analyst at XT, said:

The pound has come under some selling pressure in the wake of the Supreme Court ruling, largely due to the fact that the government doesn’t have to consult the Scottish, Welsh or Northern Irish assemblies before triggering Article 50.

The appeal itself was rejected, but it still remains likely that Mrs. May will meet her self-imposed deadline of beginning the formal Brexit process by the end of March. Sterling has been volatile over the announcement, with initial moves back above 1.25 against the dollar quickly fading and the market trading down to its lowest level of the day.