Sources connected with the former couple tell us ... J Lo told Casper it was super important for him to accompany her to Ron Perelman's Hamptons fundraiser Saturday night benefiting the Apollo Theater.
Casper wasn't having it, because he desperately wanted to watch the Conor McGregor/Nate Diaz fight in Vegas with his friends.
Casper had a great time in Vegas, hanging with Kanye and UFC announcer Bruce Buffer.
J Lo, we're told, was enraged and ended the relationship ... no discussion -- simply, we're done.
Here's the funny thing ... Casper was telling people at the fight he was going to NYC Sunday to meet up with Jennifer, who was there filming a show. He had no idea being a no-show Saturday would end things.
Our sources add this ... J Lo and Casper have "broken up" several times before and gotten back together, and they say they would not be surprised at all if it happens again.
J Lo doesn't look like she's too broken up about it at the moment, though ... she was still in New York this morning looking perky as ever.