Kathie Lee Gifford could not resist getting nasty with her old Live partner Regis Philbin after they sat down for an interview for her new gig as a television reporter on The Insider.

The National Enquirer has learned Kathie Lee ripped into him off camera over the ugly issue of why she was never brought back on the show as a guest host.

“The sparks flew between them — and not in the most heartfelt way,” said a source. “They’ve always poked fun at each other in public and in private — but off-camera some of Kathie Lee’s comments apparently turned downright nasty.”

The source believes that it’s still a sticking point with Kathie Lee that she has not been asked back on the set since she left in 2000.

“Kathie Lee is perturbed by not being invited back,” the source said. “It’s something she and Regis don’t discuss. They didn’t even have her on when she was promoting herself as a singer with her Goodnight Angel CD a year after she left the show.

“Regis has felt the show needed to distance itself from the former cohost, and allow new host Kelly Ripa to spread her wings. Plus, Regis’ pal, producer Michael Gelman, and Kathie Lee never got along well.

“That’s just an example of how Kathie Lee and Regis have never and will never see eye to eye on everything.

“It doesn’t mean they don’t care about each other, it just means they can pick each other apart whenever they get together!”

While on camera they happily reminisced about great trips they took together for the show, but then she let Reege have it when the cameras were off.

For more of this story, pick up this week’s issue of The National Enquirer.