Oprah Winfrey called her candid sit-down with Whitney Houston "the best interview I’ve ever done" – but The ENQUIRER has learned the two waged such a war of egos behind the scenes that the interview almost didn’t happen!

Insiders say bad blood has been simmering between the superstars for years over Whitney’s struggle with drugs – and their secret feud exploded off camera when the pop diva angrily blasted Oprah as a liar!

"Whitney knew Oprah’s show could help her clean up her tarnished image after years of drug use and her troubled marriage to Bobby Brown – and Oprah knew the interview could help her sagging TV ratings," says an insider.

"But Whitney felt Oprah needed her more than she needed Oprah. Whitney was contacted by various media outlets vying for the ‘big get’ interview with her during her comeback campaign."

Whitney, 46, had her pick of the litter, but chose Oprah because she wanted to appeal to her viewers and knew that if Oprah pushed her too far during the interview she was going to push back and expose some of Oprah’s own secrets!

"Whitney will never forget that when she was down and out, Oprah wanted NOTHING to do with her," an insider told The ENQUIRER.

And THAT pissed-off Whitney to no end . . .

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