The number of cigarettes sold in the United States in 2005 fell to the lowest level in 55 years largely due to enforcement of marketing restrictions imposed on the tobacco industry, the National Association of Attorneys General (NAAG) said on Wednesday.

According to federal tobacco tax figures, cigarettes sales slid 4.2 percent from 2004 levels in the largest one-year percentage decrease since 1999, the group said in a statement.

The attorneys general said 378 billion cigarettes were sold in the United States in 2005, the lowest number since 1951.

The drop continues an eight-year decline in cigarette smoking since the 1998 Master Settlement Agreement (MSA) between U.S. states and the tobacco industry that settled state lawsuits over the costs of treating smoking-related illnesses, the NAAG said.

Overall, cigarette sales have plunged more than 21 percent since the agreement, which raised cigarette prices and severely restricted industry marketing practices, the organization said.

"It is not a coincidence that cigarette sales are down and fewer people are smoking. The Master Settlement Agreement was designed to protect the public and reduce cigarette consumption — and it does just that," said Vermont Attorney General Bill Sorrell.

The major companies that signed the MSA are Philip Morris, a unit of Altria Group Inc. (MO); R.J. Reynolds Tobacco Holdings Inc. (RAI); British American Tobacco Plc's Brown & Williamson unit; and Lorillard, which trades as Carolina Group (CG) and is part of Loews Corp.

The U.S. Centers for Disease Control and Prevention considers cigarette smoking to be the leading preventable cause of death in the United States. About 440,000 people die each year from lung cancer and other diseases related to tobacco use.