From MTO: just got word of the BLOCKBUSTER REPORT of the summer. You know how the news just broke that Ciara BROKE IT OFF with rapper Future for cheating . . . well you are NOT GONNA BELIEVE who they say he was cheating with . . . RIHANNA!!! spoke with one of Ciara's CLOSE FRIENDS who tell us that Ciara suspected that Future was SLEEPING WITH her arch-nemesis RIHANNA . . . and that was enough for CIARA to END THEIR ENGAGEMENT . . . and their RELATIONSHIP.

Here's what we know (#FACTS) - Rihanna worked with Future all this summer, for a bunch of tracks for her new album. The pair spent many LATE NIGHTS in the studio together, while Ciara was at home with her baby. And it was during this time that PEOPLE STARTED WHISPERING that Rihanna and Future were having MORE THAN JUST A BUSINESS WORKING RELATIONSHIP.

And those RUMORS were reportedly CONFIRMED when photos surfaced of Rihanna and Future at a CLUB TOGETHER earlier this summer. Cici's friend told, "Future promised that he would just make music and then come back home to be with Ciara and their baby. But he was out in the club with that HEAUX [Rihanna]."

Let's make this CLEAR . . . Ciara and her girls HATE Rihanna. Not dislike . .. . HATE!!! And Future knows this, so for him to be ALLEGEDLY creeping with Ciara's ARCH-ENEMY like that, made it an impossible pill to swallow.

Ciara's girlfriend explained to, "Future is a rapper, so he's going to be messing around. It's one thing if he's with some stripper on the road, or with one of your babys mothers. I'm not saying it's right, but things happen." The snitch continued, "But to be [allegedly] f*ckin the nasty b*tch that your fiance HATES . . . while Ciara's at home taking care of your baby less than a mile away. That's some sh*t you just can't forgive."

We can't say for CERTAIN whether Future SMASHED RIHANNA. But we know that CiCi DROPPED HIM when she suspected that something was going on.