After their marriage officially ended in November 2006, Charlie Sheen and Denise Richards seemed to be model exes.

Charlie Sheen & Denise Richards : She Wants Him Back!

But on Aug. 3, a fuming Charlie, who’s now happily engaged to Brooke Mueller, lashed out at Denise, legally demanding more time with their daughters Sam, 3, and Lola, 2. Three days later, he publicly claimed that Denise requested a “donation� of his sperm to father a child! (She denied it.) But Star can now reveal the real reason he’s so angry: Since splitting with Richie Sambora in March, “Denise has mounted a full-scale campaign to get Charlie back,� says a source close to Denise. “She’s caused a lot of tension between Charlie and Brooke, apparently in hopes of breaking them up!�

After Charlie proposed to gorgeous Brooke, 29, in July, Denise, 36, “lost it!� says the insider. “I’d like to think that it’s because she wanted to reunite with the father of her kids or because deep down she loves him. But I think she really just wants to be married to a famous, wealthy man.�

So while Denise publicly congratulated the couple—and claims she sent Brooke flowers—behind the scenes, she’s tried to wreck the romance, says the source. “I heard she pulled every trick in the book, like calling their house at all hours and scaring Brook about Charlie by talking about his wild ways in the paste,� says the insider.

But Denise’s tactics may backfire, warns a friend of Charlie, 41. “He thought this nastiness would end but it hasn’t,� says the friend. “Charlie wants to end the games before he marries Brooke. He wants a fresh start.�

The couple has yet to announce a wedding date. But as another big date- the Oct. 9 custody hearing—nears, Charlie is prepared to leak more embarrassing information about his ex to end the feud, says the source. “Charlie’s willing to spend a bundle on lawyers to set Denise straight, “says his friend. “If that means letting the world know that Denise is not a sweetheart, he’s ready to do it!�