KEVIN'S AFFAIR BEHIND BRIT'S BACK

If Britney Spears doesn't have enough reasons to be glad she's dumping Kevin Federline, here's another: A source tells Star that in early October — a month before Brit served him with divorce papers — Kevin and porn-star-turned-exotic dancer Kendra Jade began a sex-fueled affair!

"Kevin and Britney were basically living apart by early October after they had a huge fight about his carousing," the source says. "Kevin was staying at a friend's apartment in the L.A. neighborhood of Studio City — and that's where he and Kendra took their friendship to another level."

'THERE WAS REAL HEAT'
When Kevin, 28, and curvaceous brunette Kendra, 29, first met through a mutual friend in Las Vegas on Dec. 5, 2005 — while Britney was visiting family in Kentwood, La. — "there was real heat between them," the source says. And as Star exclusively reported in our Jan. 23, 2006, issue, they kissed and cuddled that day in a suite at the Venetian Resort Hotel and later in the hotel's casino. "They stayed in touch afterward through their mutual friend," the source says.

And it was that friend who put them back together. "Kev and Kendra had sex at their friend's apartment multiple times, starting in early October," the source says.

Pick up this week's issue of Star for all the details!