Britney Scared Of Losing Sons To K-Fed's Girlfriend

Her new album, Circus, leaped to No. 1, and the tour built around it is a juggernaut designed to propel Britney Spears back to the top of the pop heap.

But friends fear that the big comeback could become a circus in the worst sense: Britney, 27, is terrified she’ll end up forfeiting the allegiance of her sons, Sean Preston, 3, and Jayden James, 2, to her ex Kevin Federline’s girlfriend, volleyballer Victoria Prince.

Britney was distraught, says a source close to Kevin, 30, after seeing photos of her sons in Prince’s arms at the Japanese Kitchen Steak & Seafood house in Kevin’s

hometown of Fresno, on Jan. 19.

Victoria, 27, has been “cuddling the kids and playing mommy with them,” a source tells OK!.

“Things are very serious between Victoria and Kevin,” said the source. “At first Brit was mad, then she broke down in tears.”

What will happen now? After struggling to overcome substance abuse problems and emotional instability with hopes of renewing her relationship with the boys, Britney seems sleek and energized.

Can she perform through this new pain — or will the train fly off the rails again?

“She hates the thought of this woman mothering her sons,” the source says. “She’s afraid Victoria will take her place. And it’s tearing her up inside.”