US pop star Britney Spears' lawyers in her custody battle with ex-husband Kevin Federline are quitting. 
The law firm Trope and Trope asked a Los Angeles court to be relieved as Spears' attorneys.
The firm said there has been a "breakdown" in communication with the singer that makes representing her "impossible", according to 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 visiting 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 minor collision in a car park in August. A hit-and-run charge has been dismissed.
Spears' attorney Michael Flanagan 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.