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Kevin Federline claims Britney Spears' father abused one of his grandsons, which Kevin reported to cops ... and now there's an active investigation.

Law enforcement sources tell TMZ the alleged incident went down on Aug. 24 at Jamie Spears' home in Ventura County. We're told Federline claims his 13-year-old son and Jamie got into an argument, and at some point Jamie allegedly put hands on him.

Our sources say Jamie broke down a door to get to his grandson and grabbed him once he was in the room -- however, we're told there were no visible bruises or injuries to the child. Still, the incident was serious enough to Kevin that he reported it the next to day to the Ventura Co. Sheriff's Department. We're told investigators have already interviewed witnesses and turned over their findings to the D.A.'s Office.

The D.A. will decide whether to charge Jamie.

As we first reported, Britney and Kevin just formalized a shift in their child custody agreement -- he now has their 2 boys 70% of the time. We're told that's been their informal arrangement for a year now, but they filed docs to make it official with the court.