Not only did Facebook inflate ad-watching metrics by up to 900 percent, it knew for more than a year that its average-viewership estimates were wrong and kept quiet about it, a new legal filing claims.

A group of small advertisers suing the Menlo Park social media titan alleged in the filing that Facebook “induced” advertisers to buy video ads on its platform because advertisers believed Facebook users were watching video ads for longer than they actually were.

That “unethical, unscrupulous” behavior by Facebook constituted fraud because it was “likely to deceive” advertisers, the filing alleged.

The latest allegations arose out of a lawsuit that the advertisers filed against Mark Zuckerberg-led Facebook in federal court in 2016 over alleged inflation of ad-watching metrics. The plaintiffs said in the recent filing that their new claims are based on internal Facebook documents obtained through the court process.

Facebook knew by January 2015 that its video-ad metrics had problems, and understood the nature of the issue within a few months, but sat on that information for more than a year, the plaintiffs claimed in an amended complaint filed Tuesday in U.S. District Court in Oakland.

“Even once it was decided to take action on the metrics, Facebook did not promptly fix its calculation or disclose that the calculation was wrong,” the filing alleged.

“Instead, it continued reporting miscalculated viewership metrics for another several months, as it developed a ‘no PR’ strategy to avoid drawing attention to the error.

“If Facebook had immediately corrected its miscalculation in a straightforward manner, advertisers would have seen a sudden and precipitous drop in their viewership metrics. They also would have seen that they had spent considerable money on past advertising campaigns that were not enjoying anywhere near the average viewership that Facebook had represented.”

Facebook disputed those allegations, calling the lawsuit “without merit” and saying it had filed a court motion to dismiss the fraud claims.

“Suggestions that we in any way tried to hide this issue from our partners are false,” the company said in a statement. “We told our customers about the error when we discovered it — and updated our help center to explain the issue.”

Facebook in 2016 had revealed the metrics problem, saying it had “recently discovered” it. The firm told some advertisers that it had probably overestimated the average time spent watching video ads by 60 percent to 80 percent. Tuesday’s filing alleged that Facebook had instead inflated average ad-watching time by 150 percent to 900 percent.

The plaintiffs are seeking class-action status to bring other advertisers into the legal action, plus unspecified damages. They also want the court to order a third-party audit of Facebook’s video-ad metrics.