Every day, dug-out canoes smuggle items across the Maroni river, the muddy cataract between French Guiana and Suriname. There is cocaine, mercury for illegal gold-mining and "bush meat" from endangered animals.

One form of contraband frightens Dr Gabriel Carles more than any other: heavily pregnant women. Dr Carles, a French physician, has created a world-class maternity clinic in the river town of Saint-Laurent du Maroni on the western border of French Guiana.

His clinic serves a vast territory, peopled by Creoles and the descendants of escaped African slaves, known as Bushi-Nenge. More recently, his clinic has been overwhelmed by strangers in the last stages of pregnancy, sent by people-smugglers from as far away as Haiti.

The crisis is rooted in history, and the decision to convert French colonies into overseas departements - considered provinces of France, as much part of it as Brittany or Burgundy.

Legally, Saint-Laurent - just minutes by dug-out from Suriname - lies in France. French Guiana is also part of the European Union, collecting a total of £725 million in EU subsidies for the years 2000 to 2006 - or £3,625 for each of its 200,000 residents.

Babies born in Dr Carles's clinic qualify for French citizenship once they turn 18. More immediately, they can attend school, triggering a flood of French welfare payments. Welfare levels are imported directly from France. The benefits can support an entire family, especially if that family lives in Surinam, smuggling their children across the Maroni each day for school.

Saint-Laurent has long seen a trickle of mothers from Suriname, a former Dutch colony reduced to misery by civil war. Now word of Saint-Laurent's maternity clinic has spread further afield.

Five years ago, the clinic handled 1,200 births. "Last year, 1,780 women gave birth in Saint-Laurent. That's what you would expect in a French city of 150,000 people," Dr Carles said. Officially, Saint-Laurent has 20,000 inhabitants, though the true figure is well over 30,000. Dr Carles guesses between 30 and 40 per cent of his patients are from abroad. "Before, it was cousins and relatives from the other side of the river, people wanting medical care. In the last five years it has become people trafficking."

Officials in the provincial capital, Cayenne, boast of a plan to renovate an old clinic in Albina, the Surinamese port opposite, to ease the pressure on Saint-Laurent. "But we actually started working with doctors in Albina five years ago," Dr Carles retorted. But Surinamese mothers wanted French passports, not French doctors, so no women came. By the time a formal medical pact was signed last November, co-operation had already ceased for a lack of patients, Dr Carles said.

The mayor of Saint-Laurent, Leon Bertrand, has proposed creating an international medical zone, where a baby could be born without earning citizenship. The idea is popular with local people and M Bertrand is not without clout - he is also the French tourism minister, based largely in Paris.

Dr Carles doubts the plan will succeed. "The idea is excellent. But it would require amending the French constitution. Changing the entire constitution for Saint-Laurent, that's asking a lot."

Officially, all those who cross the river Maroni must pass through Saint-Laurent's customs pier. This week, with the local car ferry away for repairs, the pier was all but deserted. Two French army speedboats roamed the river, showing the flag, but were hopelessly outnumbered by dozens of motorised dug-outs roaring back and forth, dropping passengers illegally on the river bank.

Women being smuggled to Dr Carles's clinic will sometimes head for the customs pier, where they know gallant French officers will call them an ambulance. Two months ago, a woman cut it too fine, and gave birth on the pier.

Lt Francis Pierre, a genial local gendarme commander, has 60 men to patrol 300 miles of frontier, most accessible only by river. "We try to control the flow, and slow the rate of increase," he said.

His civilian masters - damp-browed Frenchmen, wilting amid the musty grandeur of the sub-prefecture's mansion - make no pretence of believing the border can be sealed. Instead, they work to ensure new arrivals have the right papers - staving off despair with pots of excellent coffee, and a strict adherence to red tape. "There is not much that can be done. Respect the law, that's all," ventured Michel Raze, the head of mission.

Downstairs at the sub-prefecture's mansion, foreigners clamoured to renew residence permits. Across the street, a large crowd surrounded the post office, waiting for monthly welfare payments.

Angelo Calandra, the sub-prefect, gloomily contemplated the throng. "It's the jackpot for them, every month," he said, miming a tug on a fruit machine's handle. "I am responsible for an area practically equivalent to Portugal. We have so much illegal immigration here, that we are not at all certain that our official figures correspond to reality."

There are so many children in town that two new schools are being built every year. Foreigners now account for half the town's residents, and the old multi-ethnic fabric of Saint-Laurent is fraying.

Dr Carles believes crisis is at hand. "The original Creole population feels invaded, and that is starting to destabilise Guiana. I am very worried this will become another Lebanon." Locals are starting to question the historic French approach of "integrating" new arrivals - demanding instead that newcomers be denied welfare payments.

Dr Carles offered a stark warning. "The French state must make a choice that it dares not make. Either they can stop treating everybody as French citizens, educating and caring for all equally. Or they have to halt immigration into Guiana. "Guiana is an island of prosperity, surrounded by the Third World," Dr Carles said. "What is happening here in Saint-Laurent, in miniature, is what is going to happen in Europe. On a grand scale."