For a passionate Anglophile like me, the prospect of the UK leaving the EU is a disaster. A disaster for Europe, but also for Britain itself. In a world in which our continent is being slowly marginalised, a union without Britain would be paralysed down one side. This certainly holds for the Americans, because even though the "special relationship" may no longer exist, London isn't a European capital like any other. It also holds for a vast swath of the world from the Gulf to New Delhi, from Cairo to Singapore, for which European influence is historically identical with the British presence. And it holds for the Chinese, who are counting "economic divisions" the way Stalin once counted military ones, and for whom a Europe amputated in this way is seriously weakened.

But it's not just about Britain's global strategic presence. The EU needs the UK as a tireless advocate of the free market, and of competition with member states – France first and foremost – that have less of a market culture while remaining subject to protectionist and mercantilist temptations. And last, in a world where the values of freedom and democracy are the order of the day, for Europe even more than for the US, the absence of "the land of habeas corpus" would be an unfortunate symbol.

However, even without the UK, Europe will keep on going. Without Europe, on the other hand, the UK will be at a dead end. Since joining the EU Britain has pursued a very skilful policy towards Brussels: it has benefited from the single market, reduced its budgetary contribution without having convincingly explained why, and perfected the art of the opt-out. In terms of European policy, these are the equivalent of "warrants" on the financial markets – the chance of profiting from an opportunity without paying the full price. Why would the leaders of Britain, so famed for its tradition of common sense and empiricism, abandon a strategy that suits them so well in favour of a more ideology-based approach. How can they imagine that a UK, liberal and open to globalisation, would be to Europe what Hong Kong is to China?

Continental Europeans aren't altar boys. Can anybody really imagine they're going to let the most profitable activities stay solely within London, and allow the City to remain the financial capital of the euro without Britain sharing the rights and duties of the EU? Today, ambiguity is the mainstay of the City: the continentals might well put up with the capital of the euro being outside the eurozone but still in the EU. They won't – politically, economically or financially – accept it being outside the EU. There's no point even trying to imagine the UK being treated like Norway or Switzerland, keeping all the advantages of the single market. You grant advantages to countries you hope will one day join the gang, not to a country that's just slammed the door in your face.

Let the British be under no illusions. It's not the French who will be most aggressive about this, but the Germans. The French will still be attached to the idea of maintaining a little counterbalance with regard to Berlin; the Germans will continue to follow their well-trodden path of reason and power, albeit somewhat tempered. Without the UK – a traditional brake on the strengthening of European institutions – the onward march will resume more easily, and the eurozone, soon to be widened to include Poland, will integrate more quickly and become de facto Europe, aside from two or three member states.

With a "continental blockade" across the Channel, commercial agreements between London and Brussels won't be easy to negotiate – which way will the UK look? Towards the US? But Barack Obama sees the Europeans, the British included, the way we Europeans see the Swiss: slightly weary rich friends who don't create any problems for you but don't bring you any solutions either. What's the point of a UK back in splendid isolation when its preoccupations are Chinese, Asian and Pakistani? And what weight will an isolated country of 65 million people carry in the eyes of the Chinese, or even the Indians?

The wager of the British Eurosceptics is based on a double premise: taking advantage of the single market and the euro while remaining outside it; and being a "hub" of a globalised economic space. The first premise relies on the continentals being conciliatory, when in fact they'll be acrimonious. The second confuses London and the UK. Even if London maintains the extraordinary status it has invented for itself, which makes it the capital of the non-American world today, that's not going to be enough to ensure a decent standard of living in the Midlands, Wales or Scotland.

What can we call a fatal error in history? An irreversible decision based on a mistaken diagnosis with incalculable consequences. To leave would be one.

• Translated by Shaun Whiteside