Who will burn the most baby FAT faster and squeeze into a bikini– ex-Daisy Duke JESSICA SIMPSON or ex- hubby NICK LACHEY’s saucy new mom VANESSA MINNILLO?! Place your bets.   

The new moms are locked in a heavyweight battle over who will be the first to fit into a bikini, sources say.

Jessica, 32, gave birth to daughter Maxwell Drew on May 1 and is working with Weight Watchers. But Vanes­sa, 31 – whose son Camden was born Sept. 12 – is deter­mined to be the first to fit into a tiny two-piece, said an insider.

“Jessica is still struggling to drop her baby weight, and Vanessa has vowed to lose the extra pounds before she does,” the source told The ENQUIRER.

“Vanessa would never ad­mit publicly that she wants to beat Jess in a weight-loss bat­tle, but it’s her secret goal.”

As The ENQUIRER has reported, Jessica – who weighed 100 pounds when she played sexpot Daisy Duke in the 2005 film “Dukes of Hazzard” – ballooned to more than 200 pounds and was considering liposuc­tion to tighten her waistline.

The 5-foot-3 “Fashion Star” judge is now down to 155 pounds, but she’s admit­ted she was still “eating for two” a month after giving birth.

Meanwhile, Vanessa – the 5-foot-8 co-host of ABC’s “Wipeout” and a former Miss Teen USA – is using Jessica’s weight struggles as motivation, insiders say.

“Within the next few weeks, Vanessa plans to start working out with a trainer four times a week and do cardio and light weight training the other days,” said the source.

“She’s promised Nick that she’d be pushing a stroller in her smoking-hot body by Thanksgiving.

“Vanessa is very competi­tive. Not only does she want Nick to know he made the right choice when he mar­ried her, but she knows it will drive Jessica insane to see her flaunting her sexy post-baby body.”