Boris Johnson’s “get Brexit done” message resonated with an electorate tired of the endless Brexit drama, and he now finds himself leading a government with a very clear majority. However, the approach that Johnson has promised to take to Brexit virtually guarantees that the UK will be facing another dramatic Brexit crisis in 12 months’ time.

Johnson should now be able to push his withdrawal agreement through parliament, thus ensuring that the UK leaves the EU by the end of January.

Under the withdrawal agreement once the UK leaves the EU it will enter a transition phase during which the UK will have no votes in EU institutions but EU law will continue to apply, the UK will continue to pay into the EU budget and will retain access to the EU single market.

This transition period lasts until December 2020, and can be extended once by mutual agreement between the EU and UK.

No matter what Johnson claims, Brexit will most certainly not be 'done' by the end of January

The problem is that Johnson has repeatedly sworn not to seek such an extension. Trade experts are unanimous that there is no time to conclude a significant trade agreement between the EU and UK before December.

This means that the very policy designed to win the support of voters who were tired of the endless series of Brexit crises could mean that the UK could be facing a crisis in 12 months’ time as the end of the transition period approaches with no replacement trade deal in place.

Would this necessarily be a problem? After all, one of the lessons learned by commentators such as myself, who feared that insisting on the backstop would lead to a no-deal Brexit, was that there was no volte-face too shameless or too dramatic for Johnson.

If he was willing to break all his promises to the DUP, will he not inevitably be willing to seek an extension to the transition period if by December the UK is faced with the potential chaos of crashing out of the single market with no replacement deal?

Several factors

The problem is that there will be several factors at work in 2020 that will make it difficult for Johnson to pull off the kind of dramatic U-turn he pulled off in reaching an agreement with the EU in October.

It will not be possible for Johnson to wait until just before the UK crashes out and then pressure his colleagues into agreeing an extension. The withdrawal agreement between the UK and the EU that allows an extension of the transition period specifies that this extension must be requested before July 1st.

Up until February British politics will be focused on ratification of the withdrawal agreement and leaving the EU. Once that is over Johnson will have only a few months to prevail upon his colleagues to abandon their clear pledge not to extend the transition period.

Given the retirement or resignation of a large number of moderate Tory MPs, the post-election Conservative parliamentary party will have a much more hardline stance on Brexit.

Moreover, Johnson will have to persuade these hardliners to abandon their manifesto promise not to seek an extension at a time when the consequences of crashing out will still be a comfortable six months into the future.

If, having put off the hard decision, Johnson seeks a last-minute extension in December it is not clear that the EU is legally capable of agreeing to it at that stage.

The power to give the UK an arrangement such as the transition phase came from article 50, which gives the EU great flexibility to construct novel arrangements with a member state that is in the process of departing.

However, once the UK leaves the EU and enters the transition phase it is no longer a member state so article 50 is no longer available to the union.

This means that if the UK misses the July 1st deadline to ask for an extension the EU may lack the legal power to grant an extension even if it wanted to.

Tory Brexiteers

Matters are not entirely hopeless. With a large majority Johnson is free to ignore the most hardline Tory Brexiteers. Once Brexit has taken place, the heat may go out of the issue, freeing Johnson to break his “no extension” promise.

If it goes down to the wire the EU 27 may also be willing to grant a legally-dubious extension, and hope the European Court would not dare to cause chaos by striking it down.

Finally, there is perhaps time to conclude a very limited trade agreement before December, but such an agreement would be so limited that it would be similar in economic impact to a no-deal Brexit.

The one thing that is certain is that, no matter what Johnson claims, Brexit will most certainly not be “done” by the end of January.

Ronan McCrea is professor of constitutional and European law at University College London