Brexit won’t be over by 29 March 2019. Britain will legally leave the European Union on that date. But that won’t tell us what Britain’s future relationship with the bloc will be, or how closely aligned the UK will be to the EU. Those are questions for which we will have to wait for the answers.

What MPs will vote on before next March is not a ‘Brexit deal’ but a withdrawal agreement. Theresa May won’t come to the Commons and table her Chequers plan for approval, which is just as well given that she doesn’t currently have the votes to pass it. Rather, she will put forward an agreement that sets out the financial settlement between the UK and the EU, what rights EU citizens here will have and vice versa, along with the arrangements for the Irish border. The agreement will contain a statement on the future relationship but it will be a non-legally binding political declaration, thereby opening the door to a ‘blind Brexit’. For the words in the political declaration might be little more than fudge: vague commitments that are subject to interpretation and easily wriggled out of.

Mrs May had wanted a highly detailed plan. After all, if MPs are voting to hand over £39 billion to the EU, they would want to know what they’ll be getting in return. They will want to make sure they’re not paying for a trade deal that Canada and South Korea have for free.

But detail might not be helpful when it comes to getting the withdrawal agreement through Parliament. It might make it easier for Mrs May to win over her recalcitrant MPs, a task that is being taken so seriously that the chief whip is giving cabinet ministers individual lists of MPs to work on, if the political declaration is not too prescriptive. If these MPs can read it as pointing to a future relationship closer to a Canada-style free trade deal than Chequers, then they would find it easier to vote for the withdrawal agreement.

This ‘blind Brexit’ plan may get the withdrawal agreement through but it stores up problems thereafter. There really isn’t that much time between Brexit day, in March 2019, and December 2020, when the transition period ends and the new trading system is supposed to begin. Trying to address all the questions of the future trading relationship in that period will not be easy — especially when you think that a Tory leadership contest is likely to take up a chunk of that time.

The whole point of a transition was to give governments and businesses plenty of time to get ready for the new system. But now, it seems, the decision time will eat into the preparation time. In Whitehall, civil servants are already fretting about this. Nevertheless, a growing number of senior Tories are coming to the view that they just can’t take any more now, and a vague political declaration would be worth it if that would help get the withdrawal agreement over the line.

How vague? There is far from unanimity on this point. Officially, the government still says it wants proper detail. One cabinet minister tells me that the political declaration must provide ‘enough of the future framework to know we are coming into the harbour’. But it is now unlikely to go into anywhere near the level of detail that you would find in a legal text.

Another reason why the political declaration might not have the force it was once expected to have is the debate about whether Mrs May will still be prime minister as the trade deal is negotiated. There is now near universal agreement that she won’t lead the Tories into the next election; even senior Downing Street figures accept that. But there is uncertainty about when she will go — or be removed.

A large group of Tories, including a good number of cabinet ministers, think that May should leave soon after the Brexit transition starts in March next year. Others see an argument for her carrying on until nearer the next election. But many of those who formerly took that view did so because they wanted Mrs May to cling on for as long as it took for Ruth Davidson, the leader of the Scottish Tories, to come south. At Christmas, Davidson had hinted to this magazine that she might head to Westminster after the 2021 Holyrood election. But she has now ruled out ever being a candidate to be prime minister, saying she doesn’t want to take the risk with her mental health. If Davidson won’t be available, then the case for sticking with May weakens.

Some Brexiteers would argue that the certainty of ‘no deal’ is better than the ambiguity of a blind Brexit. But the problem with this is not just the diplomatic ill will and geopolitical tensions that no deal would produce: the country is just not ready for it. The government began serious preparations for it only this summer and it is hard to avoid the conclusion that it would lead to significant short-term disruption. The danger for Brexit is that this damages the Brexit brand and that voters will come to associate it with the immediate downsides that would accompany a no deal scenario.

But if the UK can leave without undue disruption, Brexit will be secure. If the UK gets a deal it is almost impossible to imagine a major party campaigning to reverse Brexit at the next election, and voters will be more open to greater divergence from the EU in future.

Brexiteers should remember that, contrary to what is often stated, time is on their side. Once Britain is out of the EU, it will become easier to argue for the UK pursuing its own path in various fields. Brexiteers will be able to argue that Britain has left and, despite what the doom-mongers said, the sky hasn’t fallen in, so the country shouldn’t be afraid of going its own way on, say, gene editing. This will be especially true if the service sector, which will likely have a more distant regulatory relationship with the EU under any Brexit deal than it does now, and than any other sector of the UK economy, flourishes in the post-Brexit era.

It took 40-odd years for Britain to get as far into the European project as it did. It is not realistic to think that all of that integration can be unravelled in 40 months. Brexiteers might not like it, but they will have to be patient. Brexit will be a process, not an event, and one with many years left to run.

A vague political declaration would be worth it if that would help get the withdrawal agreement over the line

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