A federal judge has ruled that a newly enacted New Jersey law compelled him to seriously weaken a cigarette lawsuit.

The ruling gave the tobacco industry its second important victory at the state level in seven weeks in the continuing battle over product-liability suits brought by ill and injured smokers.

The ruling, handed down by U.S. District Judge H. Lee Sarokin in Newark this week, was a partial judgment for three leading cigarette manufacturers: Liggett Group Inc., Philip Morris Inc. and Loews Theatres Inc., the successor to P. Lorillard Inc.

The issue involved a legal doctrine concerning torts, or wrongful acts. The doctrine is that when the alleged risks of certain products, including cigarettes, have been common knowledge to consumers for years, the courts cannot deem the products legally defective.

Lawyers for the plaintiff, Antonio Cippolone, had contended that the doctrine didn't apply because cigarettes are "an unreasonably unsafe product." His wife, Rose, a heavy smoker for 42 years, died of lung cancer in 1984, when she was 58.

The industry's earlier victory was in California, where a surprise alliance of trial lawyers, insurers and manufacturers submitted to the legislature a bill that would protect the industry from smokers' lawsuits. The bill passed quickly and was signed by Gov. George Deukmejian on Sept. 30.

Even before last week's ruling in Newark, industry observers considered the Cippolone case a pivotal one, partly because his lawyers have obtained numerous confidential documents dealing with sensitive issues -- such as what the defendants knew about addiction and smoking-related diseases, and when they knew it.

Last year the companies asked Sarokin to invoke the tort doctrine in the Cippolone suit. The judge refused, citing other New Jersey cases as precedents.

Meanwhile, lobbyists for the drug and tobacco industries and others launched a campaign to put the tort doctrine on the statute books.

As in California, Covington & Burling, a leading Washington law firm that represents the Tobacco Institute and pharmaceutical manufacturers, played a behind-the-scenes role. The New Jersey legislature, without full hearings, passed the law and sent it to Gov. Thomas H. Kean, who signed it on July 22.

For Sarokin, the issue was whether the legislature had intended the law to apply to cases filed on or after July 22, or to pending cases as well. The Cippolone suit was filed in 1983.

Citing a statement by the state assembly's insurance committee, Sarokin said the legislative history required him to strike the plaintiff's "unreasonably unsafe product" analysis.

Marc Z. Edell, counsel for Cippolone, said the ruling "substantially undermines our case. We were hopeful that there was going to be a full exposition of the case on the merits, but this ruling prevented that."

Edell will seek a rehearing or appeal on the ground that Sarokin misinterpreted the new law because he did not have the full legislative history before him.

Earlier, Edell lost a court battle to retain in the suit a complaint that the companies should have disclosed to smokers such health hazards as they knew about over and above the warnings on cigarette packages required by the federal Cigarette Advertising and Labeling Act.

As a result of the court defeats, Edell faces a jury trial in January with a stripped-down complaint focusing mainly on whether the companies should have disclosed health-related information known to them before the government warnings went on the packages in 1965.

A Philip Morris spokesman said, "We believe the judge's ruling was correct, and we are gratified by the result."