RELATIONS between France and Tunisia have been put under strain in recent weeks after two relatives of the Tunisian president, Zinedine Ben Ali, were placed under judicial investigation on a charge of boat-jacking.

Imed and Moaz Trabelsi, nephews of Ben Ali's wife, Leila, were interviewed in May by a French judge investigating the theft of the Beru Ma, a luxury motor yacht belonging to the president of Lazard Freres bank, Bruno Roger.

The Beru Ma - a 20m Princess V58 worth about £2 million - disappeared off the coast of Corsica in May 2006 and re-emerged a few weeks later in the Tunisian port of Sidi Bou Said, repainted and with a new registration.

French police are convinced the yacht was stolen under orders from the Trabelsi brothers, who are now under investigation for "complicity to carry out theft in an armed group".

The man alleged to have commandeered the craft - a sailor called Cedric Sermand - has told investigators he was paid the equivalent of £60,000 to steal and deliver the Beru Ma. He also implicated the Trabelsis in two other boat jackings from French ports, one from Cannes and the other from Le Lavandou.

According to Sermand, the Trabelsis used a combination of intimidation and bribery to persuade the head of customs at Sidi Bou Said to issue the boat with false papers.

"I was surprised by how easy it was to nick a boat of such value. It took me just five minutes to get it going and we were off," Sermand was quoted as telling investigators by the newspaper Le Canard Enchaine. He is expected to stand trial next year.

The affair has had major diplomatic repercussions partly because Bruno Roger is a close friend of both President Nicolas Sarkozy and his predecessor, Jacques Chirac, but also because of long-standing claims by the Tunisian opposition that members of Ben Ali's wider family are guilty of corruption.

In the past five years, 1000 boats have been stolen from marinas along the coast between Nice and Cannes. Most of these have been small pleasure craft, but they include more than 30 luxury motor yachts similar to the Beru Ma.

Thierry Cassez, of the Toulon gendarmerie, said: "For someone who knows what he is doing, one of these yachts is as easy to take as a scooter."

Sales of luxury cruisers have rocketed recently in the so-called "golden triangle" between Monaco, Cannes and Saint-Tropez, as the new super-rich from Russia and eastern Europe emulate the English and American jetsetters of earlier times. In the past decade, the number of yachts of more than 24m based along the coast has more than doubled.

Typically, the gangs infiltrate a marina during the night and break into the target boat's cockpit. In most cases, there is no alarm system and it takes little skill to hotwire the electrics in the starter-motor.

The rise in thefts has encouraged port managers and boatowners to step up security. Many motor yachts are now equipped with GPS satellite-positioners that allow police to trace their movements.

But by the time the alert is raised, the boat will have been moving for many hours. And once the stolen vessels are outside French territorial waters, police need international warrants to continue their investigations. These can take weeks to get hold of.

According to Cassez, the boats are always stolen to order - and the main culprits are gangs operating out of eastern Europe, especially Lithuania, taking boats for wealthy Russian clients.

Not all the yachts that disappear are necessarily stolen. Insurance companies automatically place investigators on any loss of more than 200,000 due to the growing incidence of fraud.

In one recent case, an owner was reimbursed the full value of his motor yacht, which was presumed stolen. But then divers chanced up on the wreck of the missing boat, with its hull pierced in several places by a pneumatic drill.