One of France's most glorious traditions, the expense-account lunch in fine restaurants, has suffered a fiscal blow from the Socialist government, and the princes of haute cuisine are groaning into their saucepans.

The cause of their stewing is a new, 30 percent tax on entertainment and other business expenses that Budget Minister Laurent Fabius says will bring almost a billion dollars a year into the state treasury to help pay for social welfare programs enacted by President Francois Mitterrand's Socialist Cabinet.

Some of France's top chefs complain that the measure also has led businessmen to cut back drastically on the amount of money their companies authorize for taking clients to lunch. As a result, they say, some of the classiest tables in a nation famous for fine eating are sitting empty from 1 p.m. to 3 p.m.

"It is catastrophic," said Gilbert Lejeune of Le Doyen, one of Paris' best known eating temples. "It is the economic ruin of the country."

Besides its stiff rate, the tax has rumpled the tempers of Parisian restaurant owners because, in their eyes, it seems directed at one of the pillars of French renown--unparalleled food.

"The very future of restaurants could be made questionable by the tax on business expenses," said Bernard Fournier, owner of Le Petit Colombier, in a letter to members of the Parisian Syndicate of Professional Restaurateurs. "It is a direct attack on the hospitality and prestige of our country's gastronomy."

Another restaurant owner said the new tax hits the entire range of services surrounding the care and feeding of prospective clients visiting Paris--from expensive restaurants to luxury hotels to entertainment.

Estimates vary on how much business the new tax actually is costing fancy restaurants. Lejeune said his lunch business is off by half at Le Doyen, down to about 140 servings a day, while evening service has slumped only slightly. Julien Francois, owner of Nicolas in Paris and president of the National Restaurateurs' Syndicate, reported a similar drop. Fournier's group surveyed 100 top Paris restaurants and found business off by 12 percent overall, with the noontime lunch business hardest hit by far.

But Jean-Claude Vrinat of Taillevent, another of Paris' most prestigious restaurants, said the new tax has changed nothing so far in his business. Without smiling, he added:

"You know, I don't think great restaurants or little cafes will be hit. But medium restauraunts that set very high prices without necessarily taking quality into account--and they are numerous--will suffer the heaviest consequences. The number of these restaurants is frightening."

In any case, it is fear of what will happen during the new year that has the restaurateurs most worried. On coming to power in June, the Socialist government immediately imposed a special 10 percent tax on business expenses, causing a first chill. Now that the 1982 budget is under way with the permanent 30 percent tax, they anticipate more cutbacks by corporate accountants.

Fournier said 60 restaurant owners in his syndicate made an informal survey of their business clients and found 90 percent planned to trim expense accounts this year by approximately as much as the tax.

"Do they want the French to become fast-food gastronomes?" he snorted.

Government officials argue that the tax is necessary not only for extra revenues, but also because under old legislation French businesses could deduct expense account expenditures from their taxes. This meant, they say, that the government was subsidizing good lunches for businessmen while facing budget deficits for health insurance and unemployment benefits.