Kevin Federline's camp has resisted signing final divorce papers out of concern over Britney Spears's recent behavior, a source close to Federline tells PEOPLE.
Under an agreement reached in March, the pair – once they sign the documents – would continue their interim arrangement of joint legal and physical custody of their two sons.
"Britney's attorneys are anxious to see Kevin sign off on a divorce," the source says. "But Kevin and his lawyer Mark Vincent Kaplan are dragging their heels out of concern over reports of Britney's post-rehab partying."
Federline's camp, the source adds, "wants to make sure that the divorce document makes it simple enough for Kevin to downsize Britney's access to her kids the next time her behavior troubles him."
Spears, 25, who left Promises rehab center in March, has been hitting the nightclub and restaurant scene recently. As PEOPLE magazine reports this week, she was spotted drinking two rum-based cocktails at Benvenuto Caffé on June 21, the same day she bought a $19.99 parakeet and domed cage.
"She had a couple of cocktails," the restaurant's maitre d', Sergio Cobin, tells PEOPLE. "She was in a great mood." (Another employee confirms she drank alcohol and "left a big tip.")
Spears's lawyer Laura Wasser, says: "I do not feel it's appropriate to comment the personal life or legal positions of my client." Federline's lawyer also didn't comment.