Britney Spears' sister is seeking more control over the star's fortune, in an increasingly tense battle over the singer's legal guardianship.

Jamie Lynn Spears already has some say over her sister's finances after being named a trustee of her estate in 2018.

In a court filing last week, she asked for control of money stored in a trust fund set up for Britney's children.

She requested the money be moved into accounts for which she is the custodian.

If approved, it would transfer the financial assets of the fund to those accounts and require a judge's approval to release them.

Britney Spears' finances are currently controlled by her father, James Spears, who has been his daughter's legal conservator for more than a decade due to concerns around her mental health.

He temporarily stepped aside from the role overseeing her personal affairs in 2019, citing his own health problems.

Some of Spears' fans believe she was forced into the arrangement, and have been running a #FreeBritney campaign.

Jamie Lynn Spears has now asked that all the assets of the SJB Revocable Trust be moved into one or more accounts with Fidelity Brokerage Services, which she controls.

The trust fund was established by the singer in 2004 to protect her children's future.

The new court filing does not explain what role it could play in the singer's protracted conservatorship case.

Such an arrangement is usually granted for individuals who are unable to make their own decisions, such as those with dementia or some other mental health difficulties.

For the last 12 years, Britney's father and attorney have managed her assets and personal life - including being able to restrict her visitors and communicate with doctors about her treatment.

That conservatorship has been extended to 2021 despite the singer's lawyers telling the court earlier this month that she is "strongly opposed" to her father remaining the sole conservator of her personal and financial well-being.

The conservatorship was set up when the star began behaving erratically in 2007 after her divorce from Kevin Federline, and she lost custody of their two children (although she has frequent visits).

In January 2019, she abruptly cancelled a Las Vegas residency before checking into a mental health facility.