Bombshell in the Diddy prosecution ... CNN destroyed the only known copy of the 2016 video in which Diddy is seen beating Cassie in the hallway of an L.A. hotel ... this according to new court documents.

Diddy's defense team and prosecutors filed a joint letter about evidence in the case. According to the defense ... CNN purchased the video showing Diddy attacking Cassie in the hallway of the InterContinental Hotel in L.A. The letter claims "CNN purchased the only known copy of the Hotel's surveillance footage, uploaded that footage into a free editing software, altered the video and then destroyed the original footage, even though it knew about and repeatedly reported about the federal investigation."

The defense claims CNN altered and edited the footage by "covering the time stamp and then changing the video sequence. It also includes speeding up the video to make it falsely appear that the actions in the video are taking place faster than they are. As a result, the CNN videos do not fairly and accurately depict the events in question."

The InterContinental Hotel has closed down in that location and we're told no other video of the incident exists.

The defense makes it clear ... they will be filing a motion to exclude the video from Diddy's upcoming trial. Diddy is not being prosecuted for the beating -- that's a state matter and the statute of limitations has run -- but prosecutors want to use it to show Diddy's alleged violent propensities.

The reality is this ... courts almost never admit video evidence where the video is edited -- judges insist on the raw footage, so there's an extremely good chance this video will never be played to a jury.