WASHINGTON — The decades-long decline in smoking by Americans has stalled for three years, the first time smoking rates have leveled off for that long since the federal government began collecting statistics more than 40 years ago.

After more than a decade of steep decline, moreover, smoking rates for high school students also have hit a plateau in the past few years and even increased a bit. This comes amid controversy over the targeting of young women by the R.J. Reynold’s Tobacco Co. with its Camel 9 cigarette — which is packaged in “hot-pink fuchsia” and claims to be “light and luscious.”

Together, the data released by the Centers for Disease Control and Prevention Thursday present a worrisome picture of smoking patterns, experts said, especially because the trend had been downward for so long.

“Anytime we are not seeing a decline, it’s a cause of real concern to us,” said Corinne Husten, head of the epidemiology branch of the CDC’s Office on Smoking and Health. “Smoking is the biggest cause of preventable disease we have, and we need to bring down the rates as quickly as we possibly can.”

According to the CDC report, about 20.8 percent of American adults are smokers — with 80 percent of them (36.3 million people) smoking every day and the rest smoking on some days. Adult smoking rates declined more than 15 percent from 1997 to 2004 but have been stubbornly unchanged since.

Husten pointed to several likely reasons for the unwelcome news. Cigarette companies have been spending billions of dollars to offset tax increases and discount their products, and funding has been cut sharply for several very successful state anti-smoking campaigns, she said.

Some anti-tobacco advocates said the Bush administration has not made tobacco control a priority and has not highlighted or promoted the issue. In part, they said, the stalled smoking rates are a result.

The relatively unchanged price of cigarettes since 2002 is considered important because more people stop smoking due to cost than for any other single reason. That is especially true of younger smokers. While a number of states have increased tobacco taxes, the federal government has not raised its rate for more than a decade, and President Bush has strongly opposed a proposal in Congress to increase the tax as way to expand the State Children’s Health Insurance Program.