WWW- Pop diva Janet Jackson’s former mother-in-law says she’s “always believed” the sexy singer secretly gave birth more than 20 years ago, but she’s never seen proof the alleged mystery kid exists.

“I’ve always believed that. Of course I do,” said Etterline DeBarge, the mom of Jackson’s first ex-husband, James DeBarge, 42. “I knew she was pregnant. She was heavy. I have an instinct. I can tell.”

But the matriarch of Motown Records’ musical DeBarge clan says Janet, then 18, never said she was expecting a child. And the Michigan granny admits she’s never seen evidence the phantom Jackson child exists.

“It’s like accusing somebody of murder when you can’t find a body,” she said. “We don’t know for sure.”

The curvy pop superstar was briefly married to James DeBarge, a singer and keyboard player in his family’s R&B group, in 1984. Janet’s father, Joe Jackson, disapproved of the union. The marriage was annulled in 1985.

For years rumors swirled that the songbird had a secret child who was given up for adoption, but the story took on new legs last week when James DeBarge’s kid brother and aspiring singer Young DeBarge, spilled the beans on a New York radio talk show. He claimed the mystery child was an 18-year-old girl named Renee who was raised by Jackson’s older sister Rebbie, but Rebbie has no such child, according to several Jackson family sources. Rebbie has two daughters, ages 35 and 28, and a son, 19.

Janet Jackson’s rep refused to confirm or deny the report, but said the singer didn’t want to comment.

Etterline DeBarge remains hopeful. “All my grandkids want to find me sooner or later,” she told the Daily News yesterday. “Maybe that will happen now.”