Government lawyers have been examining the possible legal routes that could open up for Theresa May after the supreme court delivers its verdict in the article 50 appeal.
Victory for the prime minister would deliver the simplest and most direct approach to initiating article 50 of the treaty on European Union, which formally begins the UK’s departure from the EU.
If, despite politically managed expectations, the 11 justices decide that the royal prerogative powers are sufficiently extensive, then she would not have to consult parliament and could proceed immediately to Brussels without bothering to secure parliamentary approval.
A surprise victory for the government at this late stage seems unlikely and would be met with head-scratching in No 10, which has already conceded that parliament should be consulted at the end of the Brexit process.
However, it would at least vindicate the expensive decision to appeal against the high court defeat. It could also strengthen the government’s resolve to carry out negotiations without ongoing reference to parliament.
The more widely expected outcome is that the justices, by a significant majority, follow the high court’s earlier ruling and declare that the government’s executive powers to sign international treaties, as inherited through the royal prerogative, do not extend to uprooting parliamentary legislation such as the European Communities Act of 1972, which delivered specific rights to British citizens.
After months of legal setbacks for the government on the issue, a narrow ruling from the supreme court that parliament must be consulted before article 50 is invoked would probably be met with a sigh of relief in Downing Street.
Rupert Croft, of Crofts Solicitors, who represents Fair Deal for Expats – one of the claimant groups in the case – said: “If the government loses, the supreme court may adopt the same approach as the high court and simply say that the government does not have power to trigger Brexit without parliamentary authority. The justices may want to keep clear of interfering with political matters [such as specifying the type of motion or bill required].”
Even if the judgment requires that a simple bill is passed by parliament, few are in any doubt that May has the Commons majority she needs for it to pass, and there is little chance of the House of Lords voting it down alone.
But any bill, however short, can be amended, and there will be a range of attempts to attach conditions. Labour favours procedural tweaks that can get enough Tory support to pass. The Liberal Democrats and Scottish National party will put up more symbolic resistance. Either way, this would be a bump in the road for the government, rather than a lasting obstacle.
It is less likely that the judges will attach additional conditions specifying what needs to be in a bill or stipulating that MPs and peers should have further specific roles in supervising Brexit. The justices are likely to consider that to be trespassing on purely political matters.
The devil would be in the detail and how strident the judges are in their language, but it could be seen as a sign of the courts seeking to rein in the untrammelled power of the executive in negotiating the terms of the exit as well as the principle of seeking an exit. As it is, the prime minister has already promised parliament a vote on the final shape of any Brexit deal.
One under-anticipated element that could introduce constitutional uncertainty and complexity into the government’s roadmap for triggering Brexit is the part to be played by the devolved assemblies in Edinburgh, Belfast and Cardiff.
Scottish, Northern Irish and Welsh lawyers will read the judgment closely to see whether it grants them any procedural or consultative role in the process of triggering Brexit.
The supreme court decision will test the significance of the Sewel convention, which states that if Westminster is introducing legislation on issues that have been devolved, it “normally” has to seek the consent of the devolved parliaments.
Handing the SNP leader, Nicola Sturgeon, something that looks like a veto is the nightmare scenario for a government that has a self-imposed deadline to invoke article 50 by the end of March.
Extending the principle of parliamentary approval to all regional assemblies would take far longer if May has to shepherd a bill through not just Westminster but the much more hostile Holyrood, Stormont and Welsh assemblies.
Sturgeon is fiercely opposed to leaving the EU single market and could use the process to rekindle demands for a special carve-out for Scotland. Obtaining the assent of the devolved government in Northern Ireland would be complicated even more by the absence of a government until at least 2 March, when new elections to the power-sharing executive are planned.
Since the Sewel convention is qualified by the imprecise phrase “normally”, most lawyers would be surprised if the justices find that it imposes an extra layer of obligations on the government. Barristers arguing in Belfast’s high court that the Good Friday agreement ceded constitutional sovereignty to Stormont over its future status lost their appeal in the lower court.