Sexpot JESSICA and hubby JT not *NSYNC over baby plans. WHY???

JESSICA BIEL has turned to an unlikely source in her quest to get hubby Justin Timberlake on the baby-making train – his former boy-band buddy Lance Bass!

Jessica, 31, is helping Lance plan his 2014 wedding to boyfriend Michael Turchin, but The ENQUIRER has learned she’s also using the time to get advice from him about how to convince Justin that the time is right to start a family.

“Justin keeps telling Jessica they’ve only been married for a year, and they should enjoy the time they have together before they rush into making babies,” said a friend.

 “But Jessica reminds him they’ve already been a couple for six years – and she doesn’t want to wait anymore.”

So Jessica turned to Lance, 34, who’s been pals with Justin, 32, for nearly two decades. The two were members of *NSYNC, which topped the charts with hits such as “Bye Bye Bye” and “It’s Gonna Be Me.”

“Lance told Jessica, ‘I’ve known the dude for almost 20 years, and if you stay on him, I swear he’ll cave,’” said another source.

Lance even talked to Justin himself.

“He told Justin that Jessica is serious, and she isn’t taking no for an answer,” the source continued.

While Jessica has been turning down new acting jobs, apparently in preparation for parenthood, Justin is in the midst of a 75-date concert tour that runs into early August.

“Jessica is determined to work on having a baby after Justin is off the road,” the friend added. “And thanks to Lance, she knows she’s doing the right thing in not letting the issue slide.”