Spears' finances and personal affairs have been controlled by a contentious legal agreement since 2008 after her life publicly unravelled. Her father, Jamie, was appointed her conservator.

At the request of Spears, 39, who has tried ousting her father from his role, private wealth management firm the Bessemer Trust was made the co-conservator in November.

At a hearing in Los Angeles on Thursday, Jamie lost a bid to retain the power to delegate investment powers for the estate.

Vivian Thoreen, representing Jamie, told the court her client did not want the power to delegate the estate's investment power to a third party, only to the co-conservator.

She argued there is "no intent or desire" for unequal powers between the two conservators. The objection was denied by Judge Brenda Perry.

Jamie took part virtually. Spears' mother, Lynne, did not appear.

Jamie's lawyers have argued his management of Spears' affairs saved her from financial ruin. Last year a court was told her estate was worth $US60 million.

A small gathering of #FreeBritney supporters stood outside the Stanley Mosk Courthouse.

The case is taking place against the backdrop of newly released documentary Framing Britney Spears, which looks at the circumstances that led to the establishment of the conservatorship in 2008.

The mother of two sons, she has not performed, recorded music or made media appearances since declaring a career hiatus in early 2019.

The next hearing is scheduled for March 17.

Australian Associated Press