The gloves are off for Robyn Gibson as she goes on the attack as Mel Gibson‘s soon-to-be ex-wife is out to get Oksana, sources say!

The Braveheart star’s determination for a quickie divorce has infuriated estranged wife Robyn, and she has crafted a startling new plan to sink his fast-track wedding to pregnant fiancee Oksana Grigorieva, The ENQUIRER has learned.

Hollywood’s most vicious divorce battle has taken a venomous turn with Mel’s scorned ex putting a private detective on Oksana, and vowing to dismantle his Holy Family Chapel in Agoura Hills, say sources.

According to insiders close to her, Robyn has agreed to a "bifurcated" divorce – a process in which the marriage is terminated quickly, but division of property, alimony, child support or custody arrangements, are determined later.

However, "Robyn is out to get Mel in the end," says a close family friend.

"There’s $1 billion at stake, and Robyn is planning on taking it all, including Mel’s chapel.

"Her attorneys have finally gotten through to her that the real estate alone the chapel sits on is worth millions. While she’s been a member of the chapel for years, Robyn’s involvement was all because of Mel. She’s not Catholic and she could not care less about Mel’s shrine," the friend divulged.

Despite appearances of Robyn and Mel seeming to work things out,  Robyn, 53, made a sudden about-face as she grew increasingly suspicious of 39-year-old Oksana’s motives, and the Gibson children began to resent the sexy Russkie pianist and wanna-be pop star.

"Robyn feels Oksana has a sketchy past, and that she’s using Mel. She also believes Oksana is out to get a big piece of the Gibson family’s financial pie," the friend said.

The source added: "She’s so furious that she’s hired a private investigator to follow Oksana. She wants as much dirt as possible to give her leverage if a court battle is necessary."