LISA MARIE PRESLEY is on the witness list to testify in the $40 billion Michael Jackson wrongful death civil lawsuit – and The ENQUIRER has learned she’s sitting on a powder keg of secrets about her ex-husband that could explode in court!

A source says Lisa Marie, 45, will re­veal that the late pop star feared he’d die of a drug overdose just like her father, music icon Elvis Presley, and she could spill bombshell details of Michael’s prescrip­tion drug abuse.

Lisa Marie – who was married to Michael for 20 months in the mid- ’90s – may also rip the covers off their sex life, name the biological fa­ther of Michael’s three children and explain why the couple never had a child of their own, insiders say.

After the “Thrill­er” singer died at age 50 in 2009 from an overdose of the powerful seda­tive Propofol, Lisa Marie said: “I wish I’d saved him.” She also inan interview that she suspected he was using drugs at the end of their marriage.

Now she will take the stand in the blockbuster suit that his heirs have filed against concert promoter AEG Live over their hiring of Dr. Conrad Murray, who’s been convicted in crimi­nal court of involuntary manslaughter in Mi­chael’s death.

“It’s clear that Lisa Marie was devastated over Michael’s death, and may still be, but there’s no tell­ing what she could reveal in court,” a Jackson family source told The ENQUIRER.

“In a civil trial, most any topic is fair game, and it’s possible Lisa Marie could be asked a lot of personal ques­tions about her marriage to Michael, including intimate details of their sex life. She could also be quizzed about why they didn’t have any kids togeth­er and whether Michael was even capable of fathering children. She could finally reveal the big secret behind the paternity of (Michael’s children) Prince, Paris and Blanket.

“And Lisa Marie will certainly be asked about his drug use.”

INSIDERS have claimed Lisa Marie knew about Michael’s addiction to the painkiller Demerol, and the fami­ly source said: “Lisa called (Jackson’s mother Katherine) after his death and said he often spoke about how he didn’t want to end up like Elvis.”

Attorneys may also grill Lisa Marie – who’s now married to her fourth husband, musician Michael Lock­wood – about Michael’s fixation on young boys as well as problems within the Jackson family, said the source.

“Lisa Marie had known Michael since she was seven years old,” said the source.

“Over the years, they shared inti­mate details of their lives with each other, talking about their ups and downs, joys and heartaches.

“If she testifies, she could blow the lid off his deepest secrets. Both the Jackson family and AEG are very worried about that possibility.”