Altria Group Inc.’s Philip Morris USA and other tobacco companies must defend a class-action suit filed on behalf of “light” cigarette smokers claiming $200 billion in damages.Atria shares fell the most in two years.

U.S. District Judge Jack B.Weinstein in Brooklyn, New York, ruled today that a suit filed by smokers may go forward on behalf of all such users in America. Mr. Weinstein said damages might total more than $100 billion.

The case represents the biggest potential legal liability for the industry since the American government sued tobacco companies for $280 billion in Washington in 1999. Today’s ruling may delay Altria’s plans to spin off its Kraft Foods Inc. unit, analyst Keith Patriquin said.

The spinoff is “just going to take longer,”said Mr. Patriquin, with Boston-based Loomis, Sayles & Co., which manages about $70 billion in assets including shares of Altria. “If you get class-action status certified, it could be big dollars.”

The ruling follows a series of legal victories by cigarette makers, including an August decision by the judge in the Washington case not to award any damages. In the ruling, which is being appealed, U.S. District Judge Gladys Kessler also said companies must stop using the words “light” and “low-tar” on their products by January.

The companies said they will appeal. In May 2005, the federal appeals court in New York threw out a class action that Mr. Weinstein certified on behalf of millions of American smokers claiming punitive damages against the tobacco industry.

In addition to Philip Morris, defendants include R.J. Reynolds, Brown & Williamson Tobacco Corp., Loews Corp.’s Lorillard Tobacco Co., British American Tobacco (Investments) Ltd. and Vector Group Ltd.’s Liggett Group Inc.

Altria shares fell $3.42, or 4.2%, to $78.90 at 12:10 p.m. in New York Stock exchange composite trading. Earlier they fell 5.3%, the largest percentage drop in two years.

Reynolds American Inc. shares fell $1.64 to $60.38, or 2.6%. Vector fell 23 cents to $16.49. Loews fell 29 cents to $37.71, and British American fell 15 pence to 1,455 pence in London trading.

“This judge has certified class actions frequently that are decertified upon appeal,” said Thomas Russo, who manages more than $3 billion at Lancaster, Pennsylvania-based Gardner Russo & Gardner, including shares of Kraft.

Philip Morris said it will also seek an appeal and to delay the trial.

“The company believes that the appellate court will find that today’s certification decision runs counter to the overwhelming weight of federal and state case law regarding class actions in smokers’ litigation and must be reversed,” William S.Ohlemeyer, Philip Morris associate general counsel, said in a statement.

R.J. Reynolds will seek also appeal and ask for a delay, said spokesman David Howard. “He’s made rulings like this one before, and they’ve been reversed” on appeal, he said.

Anthony J. Sebok, who teaches law at Brooklyn Law School, said the appeals court will probably agree to hear the case.

“It’s not a long trip to the Second Circuit” from Mr. Weinstein’s Brooklyn courthouse, said Sebok, referring to the Manhattan appeals court.

Mr. Weinstein said the smokers may claim damages on behalf of all American residents who smoked “light” cigarettes at any time from when they were introduced in the early 1970s until the start of trial. Mr. Weinstein set January 22 as the date for trial to begin in the case. Any potential trial might be delayed for months by an appeal.

The Schwab case, filed in 2004 and named after lead plaintiff Barbara Schwab, claims the cigarette makers violated the federal Racketeer Influenced and Corrupt Organizations Act, or RICO, by misleading American smokers into believing that “light” cigarettes are safer than higher-tar varieties.

“Light” cigarettes accounted for 85% of industry sales in 2002,the most recent year for which figures are available, according to lawyers for the smokers.

The plaintiffs, eight light-cigarette smokers from around America, claim the companies defrauded them and other smokers. The suit seeks economic damages, rather than compensation for death or disease caused by smoking. RICO permits successful plaintiffs to recover triple damages.

In a daylong hearing September 13, Mr. Weinstein questioned whether the smokers can prove they suffered the $200 billion they claim.

“You can’t allow the jury to grab a number out of thin air,” Mr. Weinstein said. “There has to be some basis rooted in reality.”