What happens when a judge gets demob-happy? The answer can be seen in the intervention by the imminently departing president of the supreme court, Lord Neuberger, in the debate over the role of the European court of justice – better known as the ECJ – once Britain leaves the EU. The government will doubtless be less than delighted at another dissenting voice in the Brexit process.

Judge calls for clarity on status of ECJ rulings in UK after Brexit Read more

The ECJ has long been a bugbear for Brexiteers, and some might say with good reason. The court has expanded its jurisdiction dramatically over time, meaning that an ever-increasing number of issues fall within its remit. It’s never been a good look for judges in Luxembourg to be able to strike down laws passed in Westminster, and the judgments coming from the ECJ are an odd mix, encompassing elements both of our own common law system and the civil law seen across the continent.

Theresa May set out a negotiating red line in her party conference speech last year that Britain would not up sticks from the EU only to remain under the ECJ’s jurisdiction. “That’s not going to happen,” she pronounced confidently.

The trouble is that, as with much of Brexit, the headline is far simpler than the specifics. As Neuberger’s intervention shows, it may be impossible to shake off the reach of the ECJ, however much the government tries.

This is not so much about a lack of political will, but a matter of legal reality. The government can hardly be said to have backtracked: the draft bill on withdrawing from the EU says that the ECJ’s rulings will no longer be supreme for any law passed after “exit day”. So far, so clear.

As for previous decisions made by the European court, this too is spelled out. Where the ECJ has decided something – and the British courts have therefore been required to apply that decision – the end result will remain binding here.

Facebook Twitter Pinterest ‘No wonder Neuberger feels that the position is invidious.’ Photograph: John Stillwell/PA

After that, things become about as simple as an Ikea flat pack. The bill, if passed in its present form, will leave lawyers scratching their heads – and billing their hours, of course – as they try to work out what it all means.

There are a number of big issues. One is what we do about decisions made by the ECJ after we leave the EU. It’s naive to suggest that these could be ignored altogether. Take a British regulation implementing EU law that was enacted prior to Brexit: what happens if the ECJ rules on what that EU law means, after we quit? Will British judges have to take the ECJ’s ruling into account, or implement the law as they see fit?

What we do know is that British courts won’t be forced to do what the ECJ says – the bill says a British court “need not have regard” to the ECJ’s judgments. Confusingly, though, it then states that the British court “may do so if it considers it appropriate to do so”.

Unless the government is clearer, it will be up to judges to decide just how much regard they must have for the ECJ. The possibilities are endless. It could be that British courts will be required to follow the ECJ unless there’s a good reason not to – or at the very least that the ECJ will be “highly persuasive” authority, meaning that the judges in Luxembourg continue to have huge sway over what happens here. At the other end of the scale, ECJ decisions might be seen as barely relevant background, largely ignored and no more important than an academic article or a decision from somewhere like Singapore.

No wonder Neuberger feels that the position is invidious. Jump one way and judges will be “enemies of the people”. Go the other and they could be seen as casting aside helpful authorities for political reasons.

Why are judges worried about the ECJ's post-Brexit role? Read more

The other big oddity is that the law will ossify from “exit day”. British judgments given in the past on the basis of ECJ jurisprudence will be confirmed as good law, even if the ECJ later changes its mind on the issue. Confused? You should be. British courts could still, in effect, be bound by decisions made by the ECJ that the ECJ has itself overturned. In that scenario, do British judges change the law to remain in line with the new European decision, or stick with what we had at the strict cut-off point?

The Brexit bill gives flexibility, but that will only lead to ambiguity. With ECJ jurisprudence no longer binding, the British courts will be free to change their mind, and it’s inevitable that over time, people will try to chip away at decisions that have been made ever since Britain first joined the EU. We may see a whole line of cases relitigated, leading to uncertainty all round.

The government will likely succeed in asserting the primacy of British courts – and that is no small change – but it needs to decide precisely what that means.

• Bobby Friedman is a journalist and barrister