BRITNEY SPEARS is preparing to move for full custody of her two sons with ex-husband KEVIN FEDERLINE - because she's reportedly concerned her boys are picking up bad habits from their dad.

The Toxic hitmaker was recently granted an extension on the equal custody agreement she has with Federline to share care of Sean Preston, four, and Jayden James, three, while she's on tour.

The former dancer was handed primary custody of the kids last year (08) after Spears suffered a mental breakdown. But the pop star is reportedly considering filing legal documents.

To make her the primary guardian when her world Circus tour comes to a stop this November (09), reports the National Enquirer.

A source says, "She is ready to fight to get her boys back. Britney feels that leaving Kevin in charge has turned her sons into cursing, ill-mannered little boys who don't behave. Britney wants to be the one who chooses where the boys go to school and where they go to church.

"She wants full custody and for Kevin to have visitation. Britney feels that Kevin is now the one with problems that the judge needs to hear about. She believes he's become a poor role model for Sean and Jayden.

The boys are used to Kevin's foul-mouthed, lazy, unemployed dancer friends, who either live at the house or hang out there. Britney doesn't want the boys growing up thinking that kind of lifestyle is acceptable." Federline, who has piled on the pounds in recent years, was this week (begs05Oct09) accused of failing to keep up with rent payments on his Tarzana, California home.