The ruling from the 9th U.S. Circuit Court of Appeals in San Francisco allows shareholders to proceed with some securities fraud claims against BP after a lower judge had dismissed them.

In March 2006, about 200,000 gallons of oil spilled from a BP pipeline onto the Alaskan tundra at Prudhoe Bay, according to the opinion.

"Despite BP's public statements suggesting that the spill was an anomaly, a second leak was discovered five months later in a different BP oil transit line in the region," the court wrote. "As a result, the company temporarily shut down regional operations."

BP-Alaska eventually pleaded guilty to a misdemeanour for negligent discharge of oil, and paid a $20 million fine to settle state and federal criminal claims, the court wrote, along with additional civil penalties.

A group of shareholders filed a proposed class action against the company in 2008, claiming that BP and its executives made knowingly false statements about the events. A Seattle federal judge dismissed all claims, but the 9th Circuit on Thursday ruled that the plaintiffs had provided enough evidence to show that some of the statements at issue should be litigated.

"In this case, facts alleged in the complaint support the conclusion that BP had been aware of corrosive conditions for over a decade, and yet chose not to address them," the court wrote.

A spokeswoman for BP Alaska, Dawn Patience, declined to immediately comment on the ruling, saying that the company had not had an opportunity yet to study it. However, she said the company had made "measurable improvements" to safety and reliability since 2006, and had "significantly increased spending on corrosion monitoring and prevention."

The case in the 9th Circuit is Claude Reese et al. vs. Robert Malone et al., 12-35260.

(Reporting by Dan Levine; Editing by Leslie Adler and Jan Paschal)

By Dan Levine