Britney Spears undergoes questioning Wednesday by Kevin Federline’s lawyers in their ongoing custody case.

A source close to the case expects Spears’s testimony, taken under oath, to last no more than one day. It begins in the morning at the offices of Federline’s attorneys.

Spears, 26, is battling ex-husband Federline, 29, over custody of sons Preston, 2 and Jayden, 1. Last month, they agreed to share time with the children on Christmas, but a final custody arrangement remains unresolved. Federline currently has primary physical custody, with supervised visitation for Spears.

It’s expected that Spears will be grilled about her past drug and alcohol use, her alleged failure to follow court orders, and anything that reveals whether she’s a fit mother.

“A deposition can be emotionally draining, because Kevin’s lawyer can ask very personal questions about her relationship with her kids – and she’ll be under oath,” explains L.A. family lawyer Lynn Soodik, who’s not involved with the case.

Federline’s lawyer, Mark Vincent Kaplan, has already completed a number of depositions of former Spears associates, including pal Alli Sims, former manager Larry Rudolph, and former bodyguard Daimon Shippen.

Spears will be accompanied to the appointment by her legal team lead by Sorrell Trope, and a court stenographer will also be present.

On Oct. 26, a court forbid her deposition from being videotaped. At that hearing, Kaplan complained that Spears had evaded a deposition four times over the course of a year.

The deposition was first reported by TMZ.com.