John Travolta and his late wife, Kelly Preston, had three children together … but their youngest kid is biologically a Presley … this according to a bombshell legal document.

Priscilla Presley‘s former business partner, Brigitte Kruse, is currently suing Priscilla’s son, Navarone Garcia, for breach of contract … but what’s way more interesting in new legal docs filed in the case is that Kruse alleges John and Kelly used Riley Keough‘s eggs to give birth to their son, Ben.

In the papers, Kruse alleges Michael Lockwood — Lisa Marie Presley‘s ex-husband — told her Kelly had been unable to bear her own children. Kruse claims John and Kelly previously used Lisa Marie’s eggs to get pregnant … but they no longer wanted Lisa’s eggs because they did not want “eggs with heroin” on them.

It’s unclear whether Lisa Marie’s eggs ever produced a child.

However, Kruse claims a deal was orchestrated where Riley — one of Lisa Marie’s daughters — gave her eggs to Travolta, so that Kelly could give birth to Ben, who is 15 now. Kruse claims Riley was given an old Jaguar and paid between $10,000 – $20,000 for the deal.

Kruse claims Lockwood was incessant, telling her John “needed to help salvage his career amid claims of sexual assault against other men.”

The docs include a handwritten note with the words “CA fertility partners,” “Ben Travolta,” and “Kelly Preston carried baby” scribbled on it.

They also include a text exchange where it looks like Ben Travolta is referred to as Priscilla’s “beautiful great-grandson.”

Priscilla’s attorneys Marty Singer and Wayne Harman tell TMZ … “After losing motion after motion in this case, and unsuccessfully seeking to have Presley’s counsel of record, Marty Singer, disqualified from representing her in this matter, Brigitte Kruse, Kevin Fialko, and their co-conspirators have demonstrated that there is no bar too low, no ethical line that they are unwilling to cross in an effort to cause further pain to Priscilla Presley and her family.”

They added … “In a completely improper effort to exert undue pressure on Presley to retract her legitimate, truthful claims, Kruse and her co-conspirators have also sued Presley’s son, cousin, and assistant. These recent outrageous allegations have absolutely nothing to do with the claims in this case. The conduct of Kruse, Fialko, and their new lawyers (they are on their fourth set of attorneys) is shameful, and it absolutely will be addressed in court.”

Jordan Matthews, of Holtz Matthews LLP, tells TMZ … “Brigitte Kruse and Kevin Fialko are heartbroken that they have been forced to file their most recent amended complaint. They have been accused of horrific acts that are not supported by any evidence whatsoever.”

Matthews says … “This amended complaint is about revealing the truth, correcting the record, and providing evidence of the work that was done in good faith to bring peace, resolution, and stability to the Presley family, despite being exposed to constant volatility.”

We reached out to a rep for Travolta … so far, no word back.