The ruling unanimously upheld a jury’s verdict that the agency was 68 percent liable for the bombing and the terrorists 32 percent liable. Under state rules, because the Port Authority’s liability was more than 50 percent, it can be forced to pay all the damages to injured survivors and to relatives of those killed.

In its decision, the court noted that the Port Authority, a bistate New York and New Jersey agency that owned the trade center, did not argue that the bombing was unforeseeable, only that it was unlikely, since its own consultants and an internal study group had predicted “with exact prescience” how an attack could be carried out.

Andrew Carboy, a lawyer for the plaintiffs, estimated on Tuesday that the Port Authority faced more than $100 million in damages for the 50 personal injury and dozen business-interruption cases remaining. Apart from its fiscal effect, the unanimous, 35-page decision signified the first time that a body of judges, versed in the nuances of the law and legal precedent as well as the evidence, had ruled on a seminal moment in New York history.

After the jury’s verdict, in 2005, the Port Authority called the notion that the agency would be more at fault than the terrorists “egregiously incorrect” and “bizarre.” The appeals court disagreed.

The amount of the damages has yet to be determined. The court, a five-judge panel of the Appellate Division of State Supreme Court in Manhattan, said that the jury decided that “the acts of these terrorists, even while obviously odious in the extreme, were not a cause for the easy absolution of this defendant from its civil obligations.” The five judges were Jonathan Lippman, Angela M. Mazzarelli, Luis A. Gonzalez, John W. Sweeny Jr. and Rolando T. Acosta.

In assigning fault, the court said, the jury considered not just the moral wrong of the terrorists, but how much the Port Authority contributed to the conditions that allowed the bombing.

“The evidence, fairly considered, clearly supported the view that the defendant’s negligence had been extraordinarily conducive of the terrorists’ conduct,” the judges said.

The blast, on Feb. 26, 1993, set off by Islamic militants who detonated explosives in a van they drove into the underground parking garage, killed six people and injured almost 1,000 others, foreshadowing the attack that brought down the towers and killed nearly 3,000 people. It created a crater about six stories deep that covered an area about half the size of a football field under the building complex.

Yet unlike its actions after the Sept. 11 attack, the federal government did not create a fund to compensate victims of the first bombing, and they have waited 15 years without a trial on damages. If Tuesday’s ruling stands, the remaining plaintiffs can go to trials for specific dollar awards.

The ruling may have at least psychological if not legal ramifications for another case now working its way through federal court in Manhattan. In that case, a small group of families of people killed in the 2001 attack declined to take federal compensation for their losses and have instead sued the airlines and airport security companies, saying that they failed to take adequate steps to protect the airplanes from hijackers. None of them have yet gone to trial.

The lead lawyer who argued against the Port Authority in the 1993 bombing trial, David Dean, said on Tuesday that he was “elated” by the appellate ruling.

“The best thing about the verdict, I think, is that it vindicated the jurors who heard the case for a month and a half and the trial court who heard the case,” Mr. Dean said.

He said that the Port Authority would have to ask permission to appeal to the Court of Appeals, the state’s highest court. If permission to appeal is not granted, he said, “I’m ready to roll up my sleeves” and start to try cases.

Steve Coleman, a Port Authority spokesman, declined to say whether the authority would appeal, and said it would continue to try to settle the remaining cases. “We have resolved all but a few dozen of the remaining cases from the 1993 bombing, and we look forward to resolving those as well,” Mr. Coleman said.

The authority said that it originally faced 575 lawsuits and that over the years, all but about 50 have been resolved. He said he did not know how much the authority had paid to settle the cases so far. The appellate court rejected the authority’s argument that it could not be held responsible because there had never been such a bombing before, noting that liability was ultimately based on “notice, not history.”

The judges said, after reviewing the evidence at the trial on liability, that there was plenty of notice. In 1984, Peter Goldmark, then the Port Authority’s executive director, recognizing the trade center’s “iconic” stature, asked Scotland Yard to assess the security of the complex and reported back to his colleagues that British officials were “appalled” that there was public parking underneath the towers.

In July 1985, an outside engineering consultant, Charles Schnabolk, issued a report saying that it was not only possible but “probable” that there would be an attempt to bomb the trade center, and that it was “highly vulnerable through the parking lot.”

The court noted that in November 1985, a Port Authority study group, the Office of Special Planning, described a scenario eerily similar to the actual bombing, in which “a time bomb-laden vehicle could be driven into the W.T.C. and parked in the public parking area.”

But the authority decided that banning public parking in the underground lot, as the report recommended, would be an unacceptable inconvenience and loss of revenue, the court said.

The decision was filled with pungent and scathing language. The judges said that the Port Authority should have realized that it faced a “potentially monstrous” risk. In one aside, they noted that the terrorists had fulfilled their mission “without meeting a scintilla of resistance.”

And they scoffed at the Port Authority’s concerns about loss of revenue from eliminating underground parking. It said that the jury heard testimony that the Port Authority’s net income from the World Trade Center at the time was about $100 million, and the loss of revenue from closing the parking lot to the public would have been “inconsequential.”

The judges said that even if they accepted the Port Authority’s argument that it was required to take only “minimal” security precautions, the authority had failed to meet even that standard.

John Sullivan contributed reporting.