The law firm Trope and Trope asked a court today to be relieved as Spears' attorneys.
The firm says there's been a "breakdown" in communication with the pop princess that makes representing her "impossible", according the filing, obtained by CelebTV.com.
A message left with a publicist for Spears' record label was not immediately returned.
Spears and Federline have been wrangling for months over custody of their sons, two-year-old Sean Preston and one-year-old Jayden James.
Federline has temporary custody because Spears, who has limited visitation rights, has defied court orders. The two were married in October 2004 and finalised their divorce in July.
On a separate legal front, an attorney for Spears wants the city attorney's office to prove that the pop star is a permanent California resident and is subject to state laws that require her to have a valid California driver's licence.
Spears faces up to a year of probation if convicted in a misdemeanour case of driving without a valid licence, a charge to which she has pleaded not guilty.
The case stems from a videotaped fender-bender in a parking lot in August. A hit-and-run charge has been dismissed.
Spears attorney Michael Flanagan earlier today requested that prosecutors be required to demonstrate that Spears, who owns homes in Louisiana and Florida, intends to make Los Angeles her permanent legal home.
"If they can establish that Britney is domiciled here in California, that she permanently intends to stay here, then she does have a requirement to get a licence," Flanagan said in an interview. "Basically, it's a fix-it ticket."
Flanagan said Spears had a valid Louisiana licence at the time of the incident and got a California licence six days after the complaint was filed.
A call seeking comment from the city attorney's office was not immediately returned. The next court date is January 25.