A decades-old feud between PAULA ABDUL and JANET JACKSON  exploded over news that Janet is vying for Paula’s old gig on “The X Factor.”

In late January, Paula, 49, announced she would not be returning as a judge for the second season of the hit FOX show, and insiders say Janet, 45, immediately put out the word that she wanted the gig.

And THAT infuriated Paula!

Although Paula had insisted in her parting statement that she harbored no bad feelings about leaving, a source says the truth was that the former “American Idol” judge was “very, very angry.”

Meanwhile, the tension between the women dates back to 1983, when Janet accused Paula of being a homewrecker for dating her still-married brother, Jackie.

The two had been best pals at the time, but the friendship collapsed when Paula, who was a cheerleader for the Los Angeles Lakers at the time, got caught in the middle of Jackie’s messy divorce.

“Janet felt responsible because it was through their friendship that Paula got to know Jackie,” explained the source. “Jackie flipped for her right away and even told his family he could see himself marrying her one day.

Things got murkier because there were rumors that Paula was dating some of the Lakers players behind Jackie’s back.

“The bottom line is Janet and the rest of the Jackson family thought Paula treated Jackie pretty badly.

“They eventually kind of made up, but things were never the same between them.”

And now that Paula has left “The X Factor,” Janet is anxious to take her spot.

“Janet would give her eyeteeth to get hired by Simon Cowell, and she’s rumored to have been in discussions with ‘X Factor’ producers for weeks,” continued the source.

“But once Paula heard about Janet’s plan, it tore open all the old wounds from the past.

“Janet’s the last person she wants to see joining ‘The X Factor.’ Paula feels Janet is now the enemy and she’s very upset. AND Janet feels she’s finally evening an old score.”