Once considered the worst kind of food sin in France, the hamburger is conquering the land of haute cuisine.

From humble fast food to Michelin-starred gourmet burgers, sales of the most quintessentially American sandwich have soared in France and the dish has become a fixture on restaurant menus.

"The French discovered the burger through (French fast-food chain) Quick or McDonald's, initially as a fatty, disreputable product," said Bernard Boutboul of food consultancy Gira Conseil.

"But in the last two or three years, so-called traditional restaurants have begun offering burgers on their menus -- because they'd be crazy not to."

Up to 80 percent of France's more than 100,000 traditional eateries are now offering burgers, he said, alongside time-honoured French classics like steak frites, duck confit and moules marinieres.

"There has been a 40 percent increase in (burger) sales in the last two years -- it's just incredible," Boutboul said.

France's newfound love of the burger was clear this week when dozens of fans lined up in central Paris for the return of US fast-food chain Burger King to the French capital.

After quitting France in 1997 because of poor sales, Burger King has returned with a vengeance and is planning to open 350-400 outlets here.

Burgers are not only increasingly popular in France, they're increasingly trendy and gone are the days when sophisticated French diners would wrinkle their noses at the idea.

Growing burger sales "come mainly from the sit-down restaurants, with a nine percent increase in the number of burgers consumed in 2013," said Maria Bertoch, a food expert at global market research company NPD Group.

"Restaurants have never been so Anglo-Saxon and the burger has really moved upmarket in recent years," said Nicolas Nouchi, the director general of food service firm CHD Expert in France.

Many top-end French eateries have jumped on the bandwagon and are now offering burger options.

The New York Times has called the burger Yannick Alleno serves at Cheval Blanc in Alps resort Courchevel one of the best in the world, and top chef Jean-Francois Piege has won praise for his "Big Burger" at chic Paris brasserie Thoumieux.

Food trucks selling burgers are particularly popular among French hipsters, and dozens regularly line up for sandwiches at mobile eatery "Le Camion qui Fume" (The Smoking Truck).

The French now each eat on average 14 hamburgers a year, putting them second behind only the British, with 17 each a year, in terms of European burger-munchers, according to NPD figures.

Most of those are still consumed at fast-food joints, with McDonald's and local chain Quick dominating the market.

McDonald's has 1,200 outlets across France while Quick has 381.

Burger King returned to France at the end of last year with one outlet at Marseille airport and another roadside restaurant near the northeastern city of Reims, and experts say it is likely to thrive.

"The market is not yet saturated... and Burger King is likely to present itself as a premium option. The battle of the burger is just beginning," Boutboul said.

Restaurants that sell burgers are even outperforming other fast-food options as the French -- hit by a stagnant economy and rising joblessness -- look for cheaper dining options.

"The burger segment is the only one that has done well in the last five years, gaining 65 million (restaurant) visits while the overall fast-food sector has lost 81 million visits. So there is growth that shows there is a place for new actors" on the market, said Christine Tartanson, Europe product director at NPD.

Both Quick, which has seen a four percent jump in sales this year, and McDonald's say they welcome the competition, with the US food giant even saying it was "thrilled" with the arrival of Burger King.