Why are we asking this now?

Defeat for the Government in a historic court battle over the right to trigger Brexit will not derail Theresa May's plans, Downing Street has said. Yet it hands Parliament the power to delay or even obstruct Britain’s departure from the EU.

What did the Supreme Court decide?

By a majority of eight judges to three, the Supreme Court found that the Prime Minister must seek the permission of MPs before she can trigger Article 50, thereby starting the two-year countdown to Brexit.

Handing down the Supreme Court judges’ ruling, which went against the Government by a majority of eight to three, Lord Neuberger said the process of Brexit would change British law and take away legal rights currently enjoyed by British citizens.

“The UK’s constitutional arrangements require such changes be clearly authorised by Parliament,” he said.

Brexit round-up: Supreme Court rules against Government

Lord Neuberger said the judgment was not about the referendum result or a comment on the merits of leaving or staying in the EU.

“The referendum is of great political significance, but the Act of Parliament authorising it did not say what would happen afterwards,” Lord Neuberger said, meaning any action taken now must be in keeping with the UK’s constitution.

What happens now?

The Government has said it will accept the ruling. It has little choice in the matter, given the Supreme Court is the highest judicial authority in the UK.

But Downing Street also insists it will not let the judgment have an impact on Ms May’s timeline for triggering Article 50.

The Prime Minister has said she wants to formally declare the UK’s intention to leave the EU by the end of March, allowing for two years of negotiations and the completion of the Brexit process in spring 2019, well in time for the 2020 general election.

In its judgment, the Supreme Court said the Government would need to table no more than a “one-line” piece of legislation in order to fulfil its legal requirement to consult Parliament.

The Government has been bracing itself for today’s judgment going against it, and so is expected to be in a position to put a bill before Parliament in early February.

Who gets a vote?

Both MPs in the House of Commons and the unelected House of Lords must approve the Government’s legislation in order for it to become an Act of Parliament and legally trigger Article 50.

Theoretically, the bill should have enough time to pass through the Commons by the February recess, at which point it will go to the Lords. The Commons will have a final vote on any amendments made in the Lords, and then it will go to The Queen for the formality of royal assent.

Supreme Court Brexit Challenge Show all 13 1 /13 Supreme Court Brexit Challenge Supreme Court Brexit Challenge People wait to enter the public gallery outside the Supreme Court ahead of the challenge against a court ruling that Theresa May's government requires parliamentary approval to start the process of leaving the European Union, in Parliament Square, central London Reuters Supreme Court Brexit Challenge Gina Miller, co-founder of investment fund SCM Private arrives at the Supreme court in London on the first day of a four-day hearing Getty Supreme Court Brexit Challenge A man waves the EU flag in front of the Supreme Court Getty Supreme Court Brexit Challenge Satirical artist Kaya Mar poses with two of his paintings in front of the Supreme Court Getty Supreme Court Brexit Challenge Pro-Europe protestors dressed as Supreme Court Justices stand outside the Supreme Court ahead of the first day of a hearing into whether Parliament's consent is required before the Brexit process can begin. The eleven Supreme Court Justices will hear the government's appeal, following the High Court's recent decision that only Parliament can trigger Article 50 Getty Supreme Court Brexit Challenge The eleven Supreme Court Justices will hear the government's appeal, following the High Court's recent decision that only Parliament can trigger Article 50 Getty Supreme Court Brexit Challenge Businesswoman Gina Miller arrives at the Supreme Court ahead of the first day of a hearing into whether Parliament's consent is required before the Brexit process can begin Getty Supreme Court Brexit Challenge Attorney General Jeremy Wright arrives at the Supreme Court in London EPA Supreme Court Brexit Challenge Protesters outside the Supreme Court in London, where the Government is appealing against a ruling that the Prime Minister must seek MPs' approval to trigger the process of taking Britain out of the European Union PA wire Supreme Court Brexit Challenge A protesters wearing a judge's wigs and robes stands outside the Supreme Court ahead of the challenge against a court ruling that Theresa May's government requires parliamentary approval to start the process of leaving the European Union, in Parliament Square, central London Reuters Supreme Court Brexit Challenge A protester holds up a placard outside the Supreme Court ahead of the challenge against a court ruling that Theresa May's government requires parliamentary approval to start the process of leaving the European Union, in Parliament Square, central London Reuters Supreme Court Brexit Challenge Pro-Europe protestors dressed as Supreme Court Justices stand outside the Supreme Court Getty Supreme Court Brexit Challenge A man waiting to enter the public gallery waves a European Union flag outside the Supreme Court ahead of the challenge against a court ruling that Theresa May's government requires parliamentary approval to start the process of leaving the European Union, in Parliament Square, central London Reuters

Could the bill fail?

In short, the answer is no. Based on a bit of political mathematics, we can say with confidence that it will pass through the Commons without much trouble.

That’s because the Conservative Party has a clear majority of MPs, and almost every Tory MP is expected to vote in line with the Government and in favour of triggering Article 50. Only the Europhile grandee Ken Clarke has said he would consider blocking Article 50.

Some Labour MPs, the Lib Dems and the SNP will probably vote against triggering Article 50. But between them they will likely command a small fraction of the MPs needed to frustrate Ms May further.

The Lords will be a more intriguing prospect. The Government has no clear majority in the upper house and many Lords were very firm supporters of Remain in the referendum.

A core of Europhiles in the Lords have been saying they will do everything in their power to block Brexit, even if that means facing accusations of being elite, out-of-touch “enemies of the people”.

Nonetheless, with the threat of a constitutional crisis and the possible triggering of a snap general election if the Lords blocks Brexit, most commentators expect the house to give the bill its reluctant assent.

How long will it take?

One sticking point in the process could come if MPs group together in a bid to win concessions from the Government over consulting Parliament during the two-year Brexit negotiations.

The SNP has already said it plans to table 50 “serious and substantive” amendments to the bill – regardless of what the Government puts forward.

Labour has also said it wants time to scrutinise the bill, though Labour leader Jeremy Corbyn has asked his MPs not to try and “frustrate” the process as a whole.

Watch the Supreme Court's Brexit ruling in full

And it is conceivable that those two, plus the Lib Dems, could convince some Remain-voting Tory MPs that there are gains to be made by blocking the bill until Ms May commits to Parliament having a greater deal of oversight across the whole Brexit process.

Essentially, the Supreme Court has opened the door for MPs to reclaim power over Brexit from the Government, in a to and fro that could take some time in the Commons.

But with the whole process likely to be given top priority in parliamentary proceedings, it could yet be done and dusted by Ms May’s March deadline.

Could the matter go to a European court?

The Supreme Court’s ruling is final, and there is no way that it could be appealed or referred any higher to the European Court of Justice.

But other aspects of Brexit are highly likely to end up being ruled on by the ECJ, according to Europe’s most senior judge Koen Lenaerts.

One key aspect which remains untested is whether or not Article 50 is revocable – can Britain back out of Brexit once the two-year countdown has begun?

The ECJ has the power to rule on things like the interpretation of the Lisbon Treaty – in which Article 50 is contained – and the validity of acts of EU institutions.