Us Magazinehas learned that as Britney Spears continues to unravel in public, Kevin Federline has been working behind the scenes to take custody of their children away from the erratic pop star.

A source tells Us that Federline, 28, and his mother, along with the help of Spears’ mother Lynne, have joined forces to aid Federline while he has cared for his sons — Sean Preston, 17 months, and Jayden James, 5 months — during their mother’s recent rehab stints, partying and hotel stays.

“He is beyond sad,” says a Federline pal. “This is the worst week of his entire life. He doesn’t even want to pick up the phone. Things are beyond not well with Britney, and he still loves her.”

Federline reportedly is now taking legal steps to protect his children. TMZ is reporting that Federline has requested an emergency custody hearing Thursday before an L.A. judge (Spears had been granted physical custody of the kids at an earlier hearing). Federline’s rep couldn’t be reached for comment, but Us has confirmed that a custody hearing has been set for Thursday.

According to our Federline insider, K-Fed made a call to his mother, Julie Bleak, on Valentine’s Day asking for her to help him take care of the children. Bleak flew to L.A. on Feb. 15 (the same day Spears, 25, checked into the Crossroads Centre rehab facility in Antigua). Our source adds that Lynne Spears has also been present for her grandchildren, taking care of the children round-the-clock with Federline.

“Britney’s family feels that Kevin has been a blessing and they are very impressed with Kevin taking so much responsibility,” says a Spears family source. “Britney’s family found out from the news this morning that Britney had left the rehab. Britney has not asked to see the kids because she knows she is not well and she does not want to scare Preston. She has no credit cards or cash. She is basically on the run at the moment.”

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