A side-debate that keeps floating into view (and floating out again) is what happens to the Calais crisis if the UK exits the EU?

The point nearly always starts with a pro-EU individual stating baldly that Brexit will end the Calais arrangement and so cause thousands of migrants to turn up at Dover claiming asylum. And they thus conclude that Brexit will actually cause more immigration problems for the UK. How nice it must feel to single-handedly turn the Brexit case on its head.

But are they right?

As always, there are several layers to such a complex situation. Let’s take the first layer.

The “Calais arrangements” mostly refer to the Le Touquet Treaty of 2003 (coming into force in 2004) which was signed by the then Home Secretary David Blunkett and his French counterpart Nicholas Sarkozy. It allowed British immigration officials and therefore passport checks on French soil and the reverse arrangement on British soil. A migrant seeking to claim asylum under the Geneva Convention of 1951 therefore becomes unable to even set foot on British territory in order to claim asylum under the convention. Thus we hear about makeshift camps in Calais where migrants get stuck.

But the important point about the Le Touquet Treaty is that it is an inter-governmental treaty between France and the UK. It has nothing to do with the EU. The fact the two states are EU members is incidental — the two states also happen to be members of NATO and other international organisations.

So the first level of argument is easily won by the Brexit side, and pro-EU types sometimes slope off at this point.

The second layer of the pro-EU argument is the suggestion that “Ah but” the French are looking for any excuse to tear up the “asymmetric” Le Touquet treaty and Brexit just might give them that excuse.

Now it’s true there are voices in France suggesting the treaty should indeed be torn up, but the arguments for doing so are generally because of the mess, the security headaches and the local political pressure it creates on the French side, leading to the French suggestion that Britain is not providing enough money and security resources. This key point is further backed by French allegations of British-sounding smugglers hanging around the region causing mischief. The point is that these issues/complaints are nothing to do with Britain’s membership (or not) of the EU — they are practical implementation issues calling for a British response. Basically more money.

And to spell it out, if the complaints are not related to EU membership then EU membership can’t be the cause of them.

Now it’s true to say that there is a key minister in the French government (Bernard Cazeneuve) and a shadow minister in the French opposition (Xavier Bertrand) who are threatening the cancellation of the Le Touquet Treaty, specifically in the event of Brexit. But in both cases, they look suspiciously like idle threats playing to pro-EU and domestic audiences. And in case we forget, the French political class are passionately pro-EU and don’t want the EU to start unwinding.

The Telegraph interview with Cazeneuve was captured as follows:

Telegraph question: “Can you confirm that either way, these [Le Touquet] accords will remain the same?”

Cazeneuve answer: “I have no intention of participating in the referendum campaign in Britain. However, it is obvious that leaving the EU will always result in countermeasures.”

Telegraph narrative: As things stand, however, he said the French had no intention of scrapping the accords.

Cazeneuve further direct answer: “Calling for the border with the English to be opened is not a responsible solution. It would send a signal to people smugglers and would lead migrants to flow to call in far greater numbers. A humanitarian disaster would ensue.”

Well quite.

The initial EUphile fear-based response is undoubtedly for the pro-EU lobby’s consumption but that’s quickly followed by the reality. Of course the French could give up on the whole treaty in a fit of pique after a Brexit vote but:

a) If that’s what EU partners will do after democracy has spoken, then why on earth are we in union with them?

b) It would ignore some of the reality of the situation and how the arrangement helps France.

And it’s the reality that keeps haunting the debate and brings us to the third layer of this argument. Namely why the French would not want to tear up this treaty.

It does after all bring British money and security resources into a zone that the French would otherwise have to police and pay for. Remember that the highly newsworthy Sangatte migrant camp pre-dated and partly inspired the subsequent Le Touquet Treaty. If the French gave up on the treaty, at the very least we’d be back to Sangatte days — there would still be a migrant camp on the Calais side, and there would still be security problems, except the French would then have to pay much more as the British would withdraw funding.

You see, two can play at that game.

As Cazeneuve himself admits (when he’s not dishing out the usual EU-fanatic fear), abandoning the Le Touquet treaty might send a “pull” signal to migrants that essentially says “it’s now easier to get to Britain so it’s a good idea to rock up at Calais”.

That’s absolutely the last thing the French want.

The other rarely-mentioned aspect surrounding France and the Le Touquet Treaty is the passport-less Schengen zone. Because as well as being a national border between Britain and France, the English Channel is also a Schengen border; Britain is outside Schengen while France is inside. And you may have noticed that the EU is looking at ways to strengthen the Schengen zone’s external border after both the mass migration event and the terrorist attacks of 2015 had prompted immediate action at state level. This constraining of Schengen could not only have the effect of bolstering the logic of the Le Touquet Treaty (as it would help the French to still station officials at key UK transit points), but looking at it from the other direction, it may also help sharply reduce the numbers of migrants able to cross the Schengen zone to Calais, thus generally relieving the political pressure on both sides of the border.

The other point to note is the nature of the Brexit agreement. A settlement that maintains the single market in an EEA-type arrangement is likely to be less controversial to the French than an all-out detachment.

But let’s just suppose that despite all of the above logic about the French maintaining the Le Touquet Treaty (and related agreements), what if they did tear up the treaty? What happens then? Would irregular migrants arrive at Dover and St Pancras station in their thousands or even tens of thousands, prompting a practical and political problem in the UK?

Based on previous history, there would be an increase but nowhere near the numbers associated with a wide open border. Because at the fourth and final layer of this argument lies an incontrovertible truth that no politician can avoid. The English Channel.

If this was a land border, then those concerns about an uncontrollable deluge of migrants could be justified. But a sea border that is 20 miles wide (at its narrowest) and traversed by licensed/commercial operators? As the Guardian noted in 2002 (pre-Le Touquet), the Eurotunnel operator stopped 18,500 migrants from crossing to the UK in the first half of 2001 alone. That’s 200 a night.

For it is the Channel that stops migrants. The Le Touquet Treaty just helps manage the side-effects.

If it came down to a tit-for-tat with the French [Really? I mean, really?], there are already ways for the British government to either sanction channel operators who wrongly bring irregular migrants to the UK and/or directly support those same operators in building better prevention measures. Because it’s easy enough to stop people at the passport gate: No money and no EU passport/no visa equals no entry.

And so it becomes the non-state actors, with or without (but very likely with) the help of the French border police guarding the already-built security fences, that would then deal with the fallout. This would again cause migrants to coalesce in camps around the port (the Sangatte area, for example) causing a new security headache for the French authorities. And who knows, after a while they may even ask for assistance from the British…which may eventually lead to the signing of an inter-governmental agreement.

The small seaside town of Le Touquet looks like a nice place to do it.