What has happened?

The High Court has ruled that the Government does not have the power to trigger Article 50 without first having a vote in parliament. Theresa May had argued that she could use Crown prerogative power to trigger the EU withdrawal process, but the High Court has ruled that the Article 50 process is irrevocable and so once initiated effectively changes rights that had been enshrined in law. The Court concluded that the EU Referendum had been advisory and that the Crown does not have the power to use its prerogative powers to override laws passed by parliament, and so parliament must decide on triggering Article 50 withdrawal.

What happens next?

The Government was given leave to appeal the decision to the Supreme Court and has announced its intention to do so. A hearing is likely to take place in early December. If the Government is unsuccessful at that stage, which many legal commentators predict, then its only legal recourse would be to appeal to the European Court of Justice. There are obvious political problems with that route. The Government has said it will make a statement in parliament on Monday which will provide more clarity on the situation.

If the ruling is upheld, what follows?

Assuming the ruling is upheld at appeal stage, or if the Government decides not to proceed with an appeal, then the Government will need to trigger Article 50 by means of an Act of Parliament. That means the Government will need to table a Bill, probably in January, that will be short and tightly drawn so as to narrow the scope for MPs and peers to lay amendments. The Government will also wish to timetable the Bill to expedite its progress.

However, there is a limit to how far the Government can control the passage of a Bill. It will need to go before both Houses and whatever its scope it will not be possible to prevent amendments being tabled altogether. The Government has a small majority in the Commons, where a number of prominent Conservative MPs have already expressed concern about the approach being taken to Brexit, and no majority in the Lords, where most peers are believed to have favoured Remain.

Could parliament derail Brexit?

In theory parliament could refuse to pass a law triggering withdrawal from the EU. However it is highly unlikely that either the Commons or the Lords would attempt such a course. The vast majority of MPs represent constituencies in which a majority voted for Leave and the Lords will be very wary about placing themselves on the wrong side of a “people versus the peers” stand-off.

Will parliament play a bigger role in Brexit?

Much more likely than attempts to block Brexit are amendments which will seek to impose conditions on the negotiations and how they take place. It is notable that Pat McFadden MP, a prominent Remain supporter and now a member of the Brexit select committee, responded to today’s judgement by saying that “Parliament should have a clear role in the substance of the Brexit negotiations, not just the process”.

That is likely to be the focus of amendments, which may seek to gain for parliamentary committees powers of oversight over the negotiations. The recent Nissan episode caused consternation among many MPs who were angered that a foreign company had more information about the Government’s plans for Brexit than the sovereign parliament.

So while it is unlikely that MPs and peers will seek to prevent Article 50 from being triggered, they will use this opportunity to put much greater pressure on the Government to publish a green or white paper outlining its fundamental vision for Britain outside of the EU. The Government has so far resisted such a move, partly because it does not want to undermine its own negotiating position but also because the moment that its position is crystallised on issues like the single market, the customs union and free movement, internal divisions will erupt.

Will this delay Brexit?

The Government continues to insist that Article 50 will be triggered by the end of March 2017. However, if the Court ruling is not overcome and an Act of Parliament is required, a delay in the timetable for triggering the withdrawal process becomes more likely. Any such delay would be embarrassing for Government but likely be welcomed by civil servants who are concerned about the Government’s readiness to begin negotiations early next year.

Does this make an early election more likely?

While there was heavy speculation over the summer that an early poll could be tempting, the Prime Minister remained resolute on sticking to an election in May 2020. At the time she stressed the importance of stability during a period of uncertainty. However, the High Court ruling has created a new uncertainty about the Government’s ability to deliver on its own plan for Brexit. If the Prime Minister found she was unable to gain parliamentary assent for triggering Article 50 in the way she wants, then she may be persuaded that an early election is necessary