Britney Spears' Dad Secretly Purchased 'Cheating' Boyfriend Video

The incriminating video showing Britney Spears' now ex-boyfriend cheating on her is now owned by none other than Britney's dad ... TMZ has learned.

TMZ broke the story ... a video has surfaced of David Lucado dancing with and kissing a woman a couple of weeks ago. A rep for Jamie Spears tells TMZ ... when Jamie got wind of the video he contacted the agency that was selling it and swiftly coughed up a SUBSTANTIAL amount of money to secure it.

We're told Jamie -- who has been Britney's conservator for over 6 years -- then called Britney and told her he had something to show her ... and it was important. When Brit saw the evidence that was all she needed ... she immediately declared the relationship over.

Jamie's rep tells us he took the action to protect his daughter from David and from the embarrassment of the video going public.

Jamie's rep says the video is under lock and key and will never be seen again.

The wild card -- we don't know if the "other woman" who secretly shot the video made other copies, but Jamie's rep tells TMZ in no uncertain terms ... if anybody posts or publishes the video he will sue them for copyright infringement.

Britney posted an Instagram video Thursday night ... saying she was having a "really, really sh***y" day."