Facebook grossly inflated video viewership metrics and then lied to advertisers about it, new court filings against the embattled social media giant allege.

The company previously admitted it played fast and loose with such statistics, but a handful of advertisers suing Facebook say the deceit happened on a grand scale.

“Facebook’s internal efforts behind the scenes reflect a company mentality of reckless indifference toward the accuracy of its metrics,” reads an amended complaint filed in California federal court Tuesday as part of a 2016 suit brought by plaintiffs including a Las Vegas ad agency and a Pennsylvania man who bought ad space on Facebook.

The plaintiffs say Facebook discovered an error in the way it calculated viewership in 2015, but swept it under the rug for more than a year.

“If Facebook had immediately corrected its miscalculation in a straightforward manner, advertisers would have seen a sudden and precipitous drop in their viewership metrics” and “would be less likely to continue buying video advertising from Facebook,” according to the amended complaint, which was obtained by the Wall Street Journal.

And when it did disclose the error to advertisers — who pay a premium based on expected viewership — the company claimed it overestimated the time users spent watching clips by 60 to 80 percent.

But Tuesday’s filings allege Facebook falsely pumped up statistics by 150 to 900 percent.

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

The company has been beset by scandal — for leaking the personal data of as many as 87 million users to political research firm Cambridge Analytica, for failing to address efforts to influence US elections, and for allowing fake news to be shared.