An American, an Englishman and a Venetian walk into a bar in St Mark’s Square. Each orders a cappuccino and a bun. The American’s tab is € 21. “Jeez,” he cries. The Englishman’s bill comes to €14. He pays up with a sigh. The Venetian looks at his bill – €7 – and calls the waiter over. “Do I look like an Italian?” he asks angrily.

It’s no joke. Bars, restaurants, cafes, shops and other service providers in Venice are routinely overcharging tourists – and in many cases by significantly more than the odd euro here and there. Last week, Venice announced the launch of a subsidised water-bus service from which tourists would be banned, a move the European Tour Operators Association angrily said exemplified the city’s “proud history of contempt towards the visitors that sustain it”. This week, I’m here to measure that contempt.

Law-enforcement officers of the Guardia di Finanza tell me they’re currently investigating more than 150 complaints of serious irregularities in city restaurants. How serious? “Serious enough that the plaintiffs took the time to visit a police station and make a formal complaint is all I can say,” confirmed one officer, citing by way of example a case in which a couple lunching in St Mark’s Square were charged £348.

Neither the mayor’s office nor the Venice tourist board cared to comment, but Franco Conte, who heads the Venice branch of the Italian consumer-protection agency Codacons, describes the practice of overcharging forestieri – or forest folk, as Venetians like to call anyone coming from beyond the lagoon – as “an epidemic”.

“By law, every bar, restaurant, cafe and hotel has to display a price list, but look around and see how many are on display. And even if you can see the tariff, they’ll add some small surcharge to the bill. And in most cases, nobody complains. Who’s going to argue over a couple of euros?”

Venetians insist that the issue of overcharging is merely a matter of perspective. “Of course we don’t raise prices for forestieri,” sighs barman Cosmo Albrizzi. “We simply lower them for locals. And that’ll be €3 [£2.10] for the spritz.” With perfect timing, my fellow barfly, a white-haired Venetian drinking exactly the same aperitif, slaps €2 (£1.40) on the bar and leaves.

And don’t think you can save a few quid by taking a picnic: since last summer, uniformed volunteers from the Office of Urban Decorum are on hand to slap £17 fines on those caught eating in public.

“Venice once was dear,” observed Byron and it still is. Take the water taxis, for instance. Or not, considering the cost. Complicated licensing laws mean the sleek, teak style statements operate as both taxis – subject to municipal fare regulation – and private hire boats. Fail to specify that you wish to take the taxi and the price of a 10-minute transfer from Piazzale Roma to La Giudecca could increase from a city-hall-sanctioned £56 to a free-market £140 or more. In Venice, there’s no such thing as a free launch.

The alternative is the vaporetto. Less romantic than the water taxi, the water bus is also a lot cheaper at just £4.20. Unless you’re Venetian, in which case it’s just 70p. Or nothing at all: a significant proportion of Venetian society fare il portoghese – a practice otherwise known as fare dodging. “There’s a €30 [£21] fine if you’re caught,” says student Luca Strellato. “But crews never challenge Venetians. Instead, they pick on the tourists.”

“Venice is dying and people are making their money while they can,” says tour guide Martino Rizzi. “In 1961 there were 170,000 people living here. Today there’s just 62,000. This summer, the last preschool in town became a hotel, and ordinary people are moving out. We’re witnessing the Disneyfication of Venice.”

Francesca Bortolotto Possati owns the Bauer – a five-star hotel in Venice – and is the director of Save Venice Inc. She both condemns and comprehends widespread overcharging.

“There are two problems,” she says. “First, there is no relationship between the city administration and those in the tourist trade. Those who run the city have little experience of business, so those who make their money from tourism are an island within an island.

“Second, there is the day-tripper situation, which is now way out of control. Last year, 540 cruise ships called here. The tourists on board spent one day in Venice, then returned to eat and sleep on the ship. Thousands more arrive on coaches, spend a few hours sightseeing and then leave. About 60,000 visitors crowd in every day in high season, but very few spend any money here and it causes deep frustration. It’s totally wrong, but businesses react by overcharging.”

How can the visitor avoid being taken for a ride on the Grand Canal? Simple, says consumer champion Franco Conte. “Always check the price before ordering anything, be it a menu, a hotel tariff or a table of taxi charges. Then learn four little words that I guarantee will save you between 30% and 50%.”

And those words? “ Fa me el conto.” It means bring me the bill. In Venetian.

Times recommends

• Oasis of the Seas: the Times review
• The 20 best new year party places to stay
• Green Spaces 2009: are you the winning writer?