In 1492, it took Christopher Columbus two months to reach uncharted land; we spent 20 minutes crashing from wave to wave in a Zodiac, or inflatable launch, belonging to the French lifeboat association.

Then one of the lifeboatmen tapped my shoulder and pointed straight ahead.

On the horizon, a low, white crescent shimmered in the sunshine: the perfect image of a desert island, save for a disappointing shortage of palm trees. "There you have it," said Pierre Becker, chairman of the Sauveteurs de Mer (lifeboatmen) of Royan, a seaside resort at the mouth of the Gironde estuary (and the place where Nicolas Sarkozy made sandcastles each summer when he was a child). "Some people spend loads of money to travel thousands of miles to the Pacific Islands. We have our own Tahiti, just off shore."

France already has twice the land area of the United Kingdom and it has just become even bigger. A new island has appeared off the French coast for, some claim, the first time in 500 years. It does not yet appear on any map or chart. It has no name. At high tide, it is about the size of two and a half football pitches. At low tide, it is at least 10 times larger. At one point last summer, the island had six or seven different kinds of vegetation and had been colonised by several types of crustacean and beetle.

A terrible storm in February cut this terra incognita in half and swept away the incipient greenery. The island with no name survived. The plants have not returned (yet). The crustaceans and beetles have.

Some sceptics suggest that "L'île Mystérieuse" (mysterious island), as it is known locally, is no mystery at all. It is a sandbank. Local environmentalists and politicians insist that it is a fully accredited island. It has existed for at least 18 months and does not vanish at high tide.

The Gironde estuary is the largest river estuary in Europe and the last to remain almost completely in a natural state. The noble sturgeon – the mother of caviar – once migrated into almost every river estuary in western Europe. It now inhabits only one, the Gironde.

From next year, this part of the French south-west coast will be part of a marine conservation park. Geologists and naturalists say that the ebb and flow of currents and coastal erosion mean that the island is likely to expand, rather than disappear. All life on earth began in the sea. The island should be protected, the ecologists say, as a natural laboratory of how, and how quickly, living organisms colonise a new scrap of land.

First of all, they say, the island needs to be protected from a ferociously invasive organism: mankind. L'île Mystérieuse is already much visited by day trippers. A sea-taxi service offers trips from Royan, four miles away. The island has been the venue for two all-night rave parties.

"Vegetation grew in the first year almost frighteningly quickly," said Jean-Marc Thirion, director of OBIOS, a local environmental group which has studied the island. "After the storm, the plants were beginning to come back but they have now been trampled out of existence by the visitors. I've seen 200 people or more on the island at one time."

When we stepped ashore on the isle with no name, we found that it was inhabited by one jellyfish, three seagulls, a couple with an expensive, bright-red barbecue, and four other picnic parties. We also discovered conclusive proof that the island is not just an island but a French island: it has an independence movement.

"All this talk of regulating and protecting this place makes me sick," said Hubert, 53, a spokesman for a large beach party of local people and Parisians. "It should belong to the people who want to use it responsibly. It is a place of liberty. It needs to be protected, yes, but from the ecologists and politicians who want to ruin it with administration and regulations and restrictions," he added.

Hubert (the de facto president of the île Mystérieuse Liberation Front) insisted that the island is not an island at all. It is just a big sandbank which has been around, in various configurations, for 15 years.

"We have been coming out here for ten years," he said. "No one dreamed of calling it an island until the ecologists decided to make work for themselves. By declaring it an island, they think that they can keep the people who enjoy it away from it. Vegetation? I saw no vegetation last year, except for a few strands of grass which were gone after a few days."

Photographs taken last summer suggest otherwise. They show clumps of "roquette de mer" (Cakile maritime) growing three or four feet high. None remains now but environmentalists believe that vegetation would return, given a chance.

Island or not, L'île Mystérieuse is a magical place: a pristine beach entirely surrounded by the sea. There is an almost perfect circle of dry, banked sand, maybe 500 metres around, littered with seaweed and pieces of wood. Arms of wetter, flatter sand stretch, lobster-like, into the distance.

A mile and a half to the south is an older island, Cordouan, which carries the grandest lighthouse in France, first built in the early 17th century. According to local historians, there was a navigational beacon on Cordouan four centuries earlier, built by Edward, Prince of Wales – "the Black Prince", heir to the throne of England and the Lord of Aquitaine for 14 years during the Hundred Years War.

A little farther south is the tip of the peninsula of the Médoc, home of the finest red wines in the world (outside Burgundy). Four miles to the east is the town of Royan. Beyond are the rolling vineyards of Cognac.

We were brought skimming – scarily and exhilaratingly – to the island by two of the most senior Royan volunteer lifeboatmen, Pierre and Eric, and by Patrick Le Guinio, president of the Royan yachting association.

"To me this is a true island," Mr Le Guinio said. "It is quite different from a sandbank. When you land here, you feel you have reached a different world. How long it will survive, I can't say. It may be only an ephemeral island, destroyed by the next big storm."

Bernard Giraud, the first assistant mayor of Royan, can see the L'île Mystérieuse from his window in the Royan town hall. It gleams on the horizon, like some kind of Shangri-La or miniature Atlantis.

"There is an ancient legend locally about islands that floated, that appeared and disappeared," he said. "It is as if this island has brought that legend alive. No one can say how long it will last, but there is every indication that it is here to stay."

The island, he believes, should be allowed to serve, partly, as a "laboratory of life" and to help scientists to discover how precisely living organisms colonise a new piece of land.

"You cannot ban people from the island outright. That would not work, certainly not in France," he said. "But we can, within the framework of the new marine park, limit the numbers and the nature of the visits. We can educate people and make them understand what a special and fragile place this is."

The last full-time keeper of the Cordouan lighthouse is about to retire. He will be replaced by guards whose duties will be to protect the rocky outcrop where the lighthouse stands. Mr Giraud hopes that they will also be given responsibility to protect the new island. If that does not work, another solution suggests itself. A distant ancestor of Her Majesty the Queen first built a light on the Courdouan rock in the 14th century. The English ruled Aquitaine long before the French.

A case surely exists that L'île Mystérieuse – or "Tahiti" or "The Sandbank" as some call it – should be declared a Crown territory just as Jersey, Guernsey, Alderney and Sark are, and therefore be, loosely, a part of the British Islands.