To a Continental thief, the right number plate promises easy pickings. Mary Dejevsky explains why you could be a target
This is a cautionary tale for anyone who hires a car in France. Recently in Arles, in the heart of Provence, we left our car in a convenient and free car park on the edge of the walled town. So did hundreds of other people, French and foreign. On returning, an hour or so later, we found a Japanese couple frantically trying to attract the attention of a passing police car. During their absence, their car had been relieved of two suitcases.

I do not know if the Japanese couple had been unwise enough to leave their suitcases visible, or had even left the car unlocked. I do know, however, that if I had been at all inclined to rob a car that afternoon, theirs is precisely the one I would have chosen.

Why? Not only because it was conspicuously shiny and new, but because its (French) registration number ended in 51. In France, as in many countries, a regional number denotes the car's place of registration. Anyone who has broken the elementary code knows, for instance, that 75 means Paris - and that Parisian drivers have a certain reputation. Personally, I have a terror of cars registered in the Var (83), which means Toulon and an area of the Riviera where the driving is unspeakable.

Until recently, 51 meant the cathedral city of Reims, and its department of Marne, and not much else. Over the past year, however, the 51 suffix has proliferated all over France. The phenomenon is particularly noticeable because French drivers move out of their home areas less than, say, British drivers, so in a particular region it is "native" plates - and the ubiquitous Parisians - that predominate, even in tourist season.

The increase in 51s has nothing to do with a mass immigration to the Marne, or a Reims baby-boom 18 years ago. It has to do with the French road tax system. Simply, Marne has one of the lowest road tax rates in France, almost half that payable in the highest-priced regions.

Your postal address dictates where your car is taxed. However, car- hire firms have some flexibility, and the biggest of them have set up offices in Reims, or the Marne's administrative centre, Chalons-en-Champagne (recently renamed from Chalons-sur-Marne for image reasons). By ensuring that their cars are taxed at Marne rates rather than Paris, Marseilles or Lyon rates, they can save large amounts of money. No fewer than 50,000 hire cars were registered 51 between January and July.

The disadvantage - for the customer - is that they are as readily identifiable as the cars which visitors used to collect at Miami airport, emblazoned with the hire company's name and, if the Arles experience is anything to go by, almost as vulnerable. For anyone in the know, a shiny new car with a 51 registration outside its home area of the Marne is very likely to be a hire car.

Given the slowness with which any administrative change is enacted in France, and the likely regional opposition to any "nationalisation" of the car tax, the government is unlikely to alter the system. French insurance companies, too, will probably see no need for any diversification of registration numbers because claims for theft from cars, as opposed to the car itself, are often made on the traveller's national travel insurance, not on the insurance taken out with the car hire.

The only possible answer at present would seem to be pressure from companies arranging fly-drive holidays to France and business trips involving car hire. And extra vigilance from customers if they pick up a hire car anywhere other than the Marne, and find that its registration number ends in 51.