Britney Spears just dumped her boyfriend of 18 months ... after finding out there's a video of him cheating on her that's being shopped around Hollywood.
We're told David Lucado is seen in the video making out with a woman and dancing with her. It looks like they're in someone's living room. We're told the video was shot in early August.
When we called Lucado ... a rep told us, "David is very, very in love with Britney. This is unfortunate and he hopes that it all works out."
Well, it didn't. Sources tell us as soon as Britney got word about the vid ... she broke up with David.
She just tweeted, "Ahhhh the single life!"