BEFORE prophesying Britain's economic doom -- a prognostic that has come to that unhappy country with the regularity of a wet Sunday -- Shakespeare's John of Gaunt described the country as a precious stone set in the Silver Sea, adding that that sea served it in the office of a wall, or as a moat defensive to a house. Britain's glory, such as it was, was to be an island cut off from the contamination of dirty farmers who ate snails and frogs and had deplorable sexual habits.

It may be said that Britain only just achieved its island status, and that the English Channel is not much of a Silver Sea. To the French it is a manche or sleeve, a fragment of a garment, not a spread cloak of blue satin like a real ocean. At its widest it is a mere 117 kilometers. At its narrowest it is a laughable 27 kilometers or, slightly more laughable, 17 miles.

It cries out to be bridged or tunneled under. A land link between united Europe and insular Britain was proposed as early as 1802, when the French engineer Mathieu suggested it to Napoleon. Presumably the French engineers would dig quietly and the infantry would break sudden cover like moles at the Dover end, much to the surprise of the English.

Isamard Brunel, who could build anything and, indeed, did, was much taken by the idea when the British and French were uneasy friends again. An Anglo-French convention to undertake the task was signed in 1875, and excavations were begun in 1882. But in 1883 the British Parliament dropped the idea as a military hazard. In 1972 England and France signed another agreement, specifically for the construction of a bored rail tunnel, but both sides lost interest until the 1980s.

Now it seems that the shovels are to come out again, and Britain's insular state is, like so many of its cherished institutions, to be liquidated forever. Tomorrow the British and French governments are expected to announce final approval for this vast and dubious enterprise.

The parliamentarians of 1883 were, of course, wrong in supposing that a channel tunnel, or chunnel, was a military danger. Invading French or Germans or lesser Slobovians using the dry route could easily be knocked on the head with cricket bats as they appeared, unseasick, on British soil. If it had already existed in 1940 the incoming Wehrmacht would have been blocked by the corpses of its vanguard. That kind of German-shooting could have been rare sport.

No, the danger comes not from war but from peace. With trains thundering under from London's Victoria Station to the Gare du Nord, and vice versa, and car drivers cursing each other from Versailles to Tooting, and vice versa, Britain will have become what she has spent a thousand years trying not to become -- namely an appendage of Europe.

Officially, she is that already, a member of the European Community that does not quite fit in, whose sausages are rejected by the French as insipid ungarlicky Boudins and whose un-iced unfrothy ale is thought to be not quite beer. Nevertheless, that stretch of water does exist and nature has contrived that it should not be too easy to pass over. There is no seasickness in the world like that of the channel crossing -- as the Normans discovered in 1066.

The bilious 17-mile sleeve cuts Britain off from a continent it has never quite understood and never much liked. When it raids Europe with murderous football fans, English ladies who speak English louder than they do at home in the belief that this turns it into a kind of Esperanto, and pimply youths vomiting up Beaujolais on the Rue de la Paix, it is fulfilling its traditional attitude toward foreigners, that of defiling them and earning their contempt.

Adry route, eliminating the foreignness that begins on a wet or airy one, will enjoin on the British a serious attempt to become Europeans. This is not a good thing for either race -- the British and the rest of the world that begins at Calais.

In practical terms the chunnel will cost a lot of money and provide a lot of work. And it will make things easier for the long-distance trucks that bring Icelandic oranges from Scotland to Provence, or Florida oysters from Lyons to Colchester. It will grant more frivolous travelers a less nauseating route to the delights and infuriations of the exotic, and it should, in about three centuries, make continental travel cheaper.

But it will insolently underline the assumption that Britain ought to keep on looking into Europe. Britain decided, under Elizabeth I and Oliver Cromwell, to look west and create America, or else to look to a real east and turn India into a sort of London suburb.

The excavation that is really needed is of a kind that will expand the channel to the dimensions of a real sea and show how isolated Britain really has to be if she is to survive. The rewriting of centuries of literature and the refashioning of the British insular brain to accommodate itself to the idea of being connected to France by a causeway will be traumatic. We need that sea all the way around with no tunnel under it.

The British are kindly people, and they do not wish to infect the folk of Europe with their own absurd culture on an endless belt of traffic. They themselves are capable of reverse infection. That is what is meant by being an island race.