The borderlands north of the frontier between France and Spain are enjoying an autumn heatwave. Holidaymakers and locals are thronging the beaches. In Biarritz, surfers are cresting the waves. The bars and restaurants are crowded. It is as if all is for the best in this best of all possible worlds.

But look a little deeper. Events in Spanish Catalonia are being watched with avid attention by the 400,000 inhabitants of the Pyrénées-Orientales, a French department centred on the city of Perpignan, who are every bit as Catalan as their southern neighbours. Should the Spanish Catalans achieve their independence, how long before the demand grows for reunification of a nation that existed as a European power for centuries until it was finally carved up by Paris and Madrid in 1659?

One thing is clear: if you think the Spanish government of Mariano Rajoy is adamant in its rejection of Catalan separatism, wait until the French are dragged into the equation. France would no more concede a millimetre of its territory than it would accept a plain woman as a Cabinet minister. So far, President Macron has said very little. His plan is to leave the dirty work to Rajoy. But if ever Spain should cave in to the separatists and the contagion should spread, the French President can be expected to spell it out for his Catalan fellow citizens: Non! Non! Et encore une fois, non. Or as they say in Catalan, No!

As it happens, there is as yet no obvious unrest in the three comarques, or counties, that make up northern Catalonia. Frenchification has worked its magic down the years, so that most people living in the shadow of the eastern Pyrenees think of themselves primarily as French. Yet it is estimated that up to one third of the local population speaks Catalan at home and that another fifth or so have at least a smattering of the language. If their Spanish confreres do somehow beat the odds and establish a national capital in Barcelona, there will inevitably be an upsurge of patriotic fervour from Cerbères (Cervera de la Marenda) to Catalan-speaking Andorra, a nominally independent country under the joint suzerainty of the Spanish bishop of Urgell and his co-prince, no less than Emmanuel Macron.

At the other end of the Pyrenees, in the Basque Country, similar pressures are building. Eighty per cent of the territory of ancient Euskadi is reposed in Spain, radiating out from the cities of Bilbao (Bilbo), San Sebastián (Donostia) and Vitoria-Gasteiz. It is here that most Basques live, some 2.2 million of them if you exclude Pamplona (Iruña) and its hinterland, now deemed to be part of the modern province of Navarre. But north of the border with France lies the Pays Basque, extending from Hendaye (where Hitler met Franco in 1940) north to Biarritz and Bayonne and east to Pau.

The population of the Pays Basque is only 300,000, but the territory is spectacular – very much part of the patrimoine of France. The extraordinary Basque language straddles the frontier, along with the rapid-fire sport of pelota and a hearty local cuisine. French basques are less strident than their Spanish counterparts. While in the past they sometimes offered sanctuary to Eta terrorists on the run from the Spanish police, only rarely did they take up arms themselves. There is little doubt, though, that a rebirth of separatist sentiment in Bilbao would be echoed by a shift in the mood of the citizens of Biarritz and Bayonne.

In that event, Macron and Paris would be anxious to nip the situation in the bud. The idea of groups of ingrates attempting to make a dent in the Hexagon is anathema to France’s amour propre. Just about everyone, from Marxists, Socialists, Republicans and the National Front, would unite to defend the integrity of the nation. Where would it end? An independent Brittany? The restoration of Nice to Italy? Shifting frontiers in Flanders and Alsace? No. It’s not going to happen. It really isn’t.

But while Macron has expressed his confidence in the ability of Mariano Rajoy to defend the interests of Spain, he knows that independence for the larger part of Catalonia is at least a possibility and that even if the “insurrection” is put down the demand for separation will fester for years to come.

So far, the contamination has been contained. Southern France is continuing to enjoy its Indian summer, while in the nation as a whole the principal concern among citizens is jobs, the economy and the President’s determined drive to reform the labour laws. On neither side of the French Pyrenees have there been significant demonstrations of solidarity with either an independent Catalonia or a unified Euskadi.

Events, however, can move with great speed and no one can predict what will happen if Carles Puigdemont, the Spanish Catalan leader, succeeds, against all political logic, in re-establishing his fractured country as an independent state.

We know what Nicola Sturgeon and the SNP think. They are with Puigdemont every step of the way, as is Sinn Fein, albeit that Gerry Adams, with no apparent awareness of the irony, has urged both sides to reach a peaceful solution. We also know that the EU feels it necessary to side with the government in Madrid, representing, as it does, an important member state. And we know what the people of Spain, outside of Catalonia and the Basque Country, think – that separatism should be put down much like a rabid dog.

But what of France? The fine weather will not last forever.