NEW YORK—A federal appeals court has upheld a jury verdict that the Sept. 11, 2001, terrorist attack on the World Trade Center was a single occurrence under the terms of the Willis Group Holdings P.L.C. policy form agreed to by the lead insurer, Swiss Reinsurance Co.

WTC leaseholder Silverstein Properties Inc. had argued that destruction of the Twin Towers was caused by two aircraft, each was a separate occurrence under its coverage and, therefore, it was entitled to two policy limits of $3.55 billion each.

In the first phase of the litigation in 2004, a jury found that nearly half of Silverstein's two dozen property insurers, including Swiss Re, were liable for only one occurrence limit. A separate jury in the second phase of the trial concluded that nine of the insurers could be held liable for two occurrences.

The U.S. Court of Appeals for the 2nd Circuit in New York last week upheld both verdicts, including the one that Swiss Re was liable only for one occurrence limit.

"Silverstein's two-occurrence claim against Swiss Re gave rise to five years of protracted litigation," said Jacques Dubois, chief executive officer of Swiss Re America Holding Corp., in a statement. The decision "resolves Silverstein's two-occurrence claim once-and-for-all in Swiss Re's favor."

Under the ruling, Swiss Re's payment obligation is subject to the terms and conditions of the operative Willis policy form and cannot exceed Swiss Re's 25% share of the $3.5 billion loss-limit Silverstein purchased, Swiss Re said.

Silverstein's insurance program for the WTC complex, which was placed through London-based broker Willis, had not been finalized at the time of the terrorist attack.

While Silverstein had sought about $7 billion from his insurers, it is entitled to approximately $4.6 billion under the appeals court ruling.

Separately last week in Manhattan district court, U.S. District Court Judge Alvin K. Hellerstein ruled that a consolidated class action lawsuit representing about 3,000 workers--who claim negligence by New York City and contractors in preventing health problems stemming from recovery and other efforts at Ground Zero--could move forward.

The judge allowed the case to proceed despite arguments by New York and its contractors that they should be given legal immunity to workers' suits.

The judge, however, did dismiss complaints against Consolidated Edison Co. and the Silverstein entities.

In an e-mailed statement, the WTC Captive Insurance Co. said: "The Captive continues to be mindful of our primary duty to our insureds. In fact, our sole obligation under our insurance policy is to those insureds."