There used to be a time when Italy's super-rich gravitated to the smartest enclaves of Sardinia for a summer of relaxation and luxury. Not any more. In an increasingly austerity-conscious country, the yacht-owning classes are coming under increasing and unwelcome scrutiny, some of which would not look out of place in a scene from the film Apocalypse Now.

"We first spotted the targets with the helicopter's radar and closed in to identify about 50 boats off the two islands," said Italian coastguard captain Pietro Mele, describing a recent raid on yachts suspected of straying too close to the coast. Swooping in, the helicopter crew barked orders to the plush pleasure craft through a loudspeaker, telling them to move on from the protected Sardinian islands of Soffi and Mortorio, where anchoring is strictly forbidden.

It was hardly an act of war, but the tough measures have been enough to spark a furious reaction from royalty, actors and entrepreneurs who keep their yachts on the nearby Costa Smeralda and are now threatening to move on to Corsica or France's Côte d'Azur.

"These helicopter raids are a huge problem and many yachts won't be back to Sardinia any time soon," said Roberto Azzi, who runs a rental agency called Emerald Yachts.

The helicopter raids top a terrible summer for Italy's upmarket sailors, who have suffered regular visits from tax inspectors instituting prime minister Mario Monti's crackdown on tax avoidance. "We are 30% down this season," complained Azzi.

The two islands have long been favourite spots for boats heading out from the swish port of Porto Rotondo, which is popular with rich Italians, Russian magnates and Arab sheikhs. Among those up in arms about being turfed out were German and Italian princes, as well as former Formu la 1 racing driver Alessandro Nannini. "Yachtsmen pass the word around and get out before the checks start," he told Corriere della Sera. "But that's got to stop. Something has to change, or I am not going back."

Azzi admitted that the islands have long been officially out of bounds, but the boats had flocked there anyway. "And now, with the crisis on, they go and send in a helicopter? The rules are arbitrary and there are 2,000 people employed in the business here who stand to lose their jobs if the yachts flee.

"The anchoring damages the sea grass, but there is a group who ignore the ban," responded a spokesman for the Maddalena archipelago national park, which manages the islands.

"They know the rules, but we are in Italy; they are on holiday," said Mele, who broadcast the message, "You are in a forbidden area, please move on," from the coastguard helicopter to the sunbathing tycoons. "Unfortunately some thought we would go away and did not move, so we sent a patrol vessel in as well," he said.

The Sardinian coastguard have a reputation for fearing no one. Last summer Steven Spielberg was fined for sailing too close to a Sardinian beach with his engines on.

Azzi said that the helicopter raid was not the only way Italian law enforcers were managing to upset the rich this summer. "Since the sinking of the cruise ship Costa Concordia, there has been a ban on cruise ships sailing too close to protected areas," he said.

The problem, he added, is that police have been confusing mega-yachts with cruise ships and harassing billionaires as they lie at anchor in a beauty spot: "They ordered Roman Abramovich's yacht to retreat from the Costa Smeralda and return to Porto Fino."

As the mega-yachts sailed by the wealthy Russians fall foul of the law, the smaller motorboats run by Italians are proving a magnet for tax police inspectors who are sent out by the Monti government to crack down on tax dodgers and raise crucial funds for Italy's battered balance sheet. "We are in a state of war" against tax evasion, Monti warned on Friday.

With owners of large motor boats who declare suspiciously low incomes coming under special scrutiny, a growing number are claiming that their boats have sunk and are putting in insurance claims, said Ugo Vittori, the head of investigative agency Eagle Keeper.

"Sinkings need to happen in deep water, for example off the Puglian coast and near the Tuscan Formiche islands," he said. "The owners say it was a miracle they survived and we send down a robot sub to find the boat, but it often turns out they have been sold in Miami or Martinique," he added.

After tax inspectors swept through the Alpine resort of Cortina d'Ampezzo this winter, ostenstatious displays of wealth have fallen out of fashion this summer and people are rediscovering mountain walking, wrote Italian daily La Stampa.

Motor racing tycoon Flavio Briatore has even threatened to close down Billionaire, his Costa Smeralda nightclub frequented by Naomi Campbell and Silvio Berlusconi, claiming: "Italy is now a country where, if you own a yacht and tie up in port, you are either a bandit or a thief."