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. She was spotted drinking two rum-based cocktails at Benvenuto Caffe 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, says. "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. Meanwhile, the producers of Cyndi Lauper's True Colors Tour say that Spears will not be appearing, after all. Spears choreographer had said she'd be performing in the Los Angeles show on June 30.
"We invited Britney to join us in Las Vegas for the tour kickoff but never heard back from her representatives. We still haven't heard anything from them," tour producer Jonny Podell says. "Of course we wish we'd been contacted, but unfortunately, at this point, we wouldn't be able to accommodate another artist in the L.A. schedule anyway."
Lisa Barbaris, Lauper's manager and another tour producer, adds that media reports over performance disagreements with Spears's camp were not true. "The rumors about Cyndi not letting Britney perform due to any performance or lip-sync issues are silly and completely false," says Barbaris.