The U.S. Patent and Trademark Office issued a preliminary ruling against Apple's "pinch-to-zoom" patent, which Apple used as a cornerstone of its case against Samsung.

The agency rejected all claims in Apple's patent on Wednesday, according to a document issued by the patent office that was filed by Samsung in federal court in San Jose, Calif. The document lists portions of the patent that were struck down on reexamination, on the basis that prior patents covered the same inventions.

The patent office said that in order for the Apple's claims to be valid, they must be considered patentable despite any "prior art patent and printed publication cited." In a detailed report, the agency said they did not.

Christal Sheppard, an assistant professor at the University of Nebraska College of Law, said Apple can still appeal if the patent office decision becomes final, and potentially salvage its patent. But if it is unsuccessful, the decision could impact the damages Apple is awarded.

"There may need to be a new phase of the trial to figure out what the damages are," Ms. Sheppard said, but not the facts of whether Samsung infringed Apple's other patents as well. "The facts of whether they infringe or not have been found."

Read more on Apple's legal options at The Wall Street Journal.