A group of the world's leading cultural experts have launched a stinging attack on the Italian government over the use of giant advertisements placed on some of Venice's most historic sights.

The directors of the British Museum, the V&A and the Museum of Modern Art in New York are among the signatories of a letter demanding that Italy's culture minister, Sandro Bondi, outlaw the billboards, which "hit you in the eye and ruin your experience of one of the most beautiful creations of humankind".

The letter, published in the Art Newspaper, and which was also signed by architect Norman Foster and the heads of museums in Boston, Dresden, Stockholm and the Hermitage in St Petersburg, accuses the Italian government of violating Venice's Unesco ranking as a world heritage site.

Facing a funding crunch as masonry began to crack and tumble from some of its ancient palazzi, Venice began to offer advertising space in 2008 on the awnings used to cover scaffolding erected for restoration work around St Mark's Square and up and down the Grand Canal.

But local conservationists said using the advertisements to pay for essential maintenance got out of hand this summer when a Coca-Cola advertisement all but obliterated views of the Bridge of Sighs behind the Palazzo Ducale. Now dubbed by locals the "bridge of signs", and rented out for €40,000 a month, the structure bears a large advertisement for the luxury goods retailer Bulgari.

"When we pass down there, tourists ask, 'Where is Palazzo Ducale?' " said Aldo Reato, head of the Venice Ente Gondola Association. "What can we say? It is hidden under all those ads."

Adding insult to injury, the council has now given permission for the advertisements to be lit at night. "Their scale dwarfs the fine detail and proportions of the buildings, and now that they are also illuminated, you cannot escape them even by night, when they are the hardest, brightest lights in town by far," the cultural experts stated in their letter.

The letter was published under the auspices of the Venice in Peril Fund, the British organisation that supports preservation work in Venice. "There is now a pool of light around the tennis court-sized ad at Santa Maria della Salute," said chairwoman Anna Somers Cocks. "They take away the wonderful darkness of Venice."

The advertisements are probably illegal, she said, citing an Italian law that forbids them if they damage "the appearance, decorum and public enjoyment" of historic public buildings.

Word of the billboards spread among the international architectural community thanks to Venice's architecture Biennale last month, said Somers Cocks. "Frank Gehry, Rem Koolhaas and Norman Foster visited and were all aghast," she said.

Bondi, the culture minister, was informed on Friday of the letter, but declined to give a response, stating prior commitments left him no time to do so.

Venice's mayor, Giorgio Orsoni, has lashed out at critics of the ads, saying he was "amazed at the complaints... The awnings are there for safety and I do not see why, just like in other Italian cities, they cannot be used for advertising." Orsoni's aides point out that the millions of euros needed for work are no longer being handed over by Rome.

A spokesman for the mayor told the Observer that "Venice, which is obliged to maintain these precious monuments, is forced to adopt this system". The €2.8m (£2.4m) now being spent restoring the Palazzo Ducale and Bridge of Sighs will be entirely covered by advertising revenue.

But the Venice in Peril experts are not convinced. "To those who say that the money the advertisements bring is necessary to restore those buildings, we remind you that after the great flood of 1966, when Venice was in a much worse state and Italy a much less rich country, no one contemplated using this method to raise funds," they wrote. "Other ways of financing restoration must be found, otherwise Venice is doomed to be covered in ads for the rest of its life because its buildings will always be undergoing work due to their great age and the environmental fragility of the city."

One solution has been offered by Roberto Ciambetti, the regional authority's public spending assessor, who proposed that assets seized from mafia godfathers be sold off to pay for Venice's upkeep.

The city of Florence, meanwhile, has fought back against intrusive advertising. The council let a supermarket chain hang a large advertisement on the Ponte Vecchio last year in return for supporting restoration work, before a public outcry forced the mayor to tear it down and the supermarket to apologise. "No company sponsoring a concert would get its jingles played in the middle of a Mozart symphony," said Venice conservationist Nelli-Elena Vanzan Marchini.

"We ask you to imagine the disappointment that the 17.5 million visitors to Venice this year will feel," the Venice in Peril letter stated. "They come to this iconic city with an image of it in their mind's eye, and instead they see its famous views grotesquely defaced."

Somers Cocks pointed out that almost every view in Venice has been represented in art. "You know what it should look like, so it is shocking when you get there," she said.