OK! has learned exclusively why Britney Spears has chosen to NOT show up for today’s deposition in her ongoing custody battle with Kevin Federline.

So why the absence? Brit’s attorneys showed up to the deposition on time, apparently expecting their client to follow! She finally called in "sick" about an hour later. "She’s very sick with high anxiety," explains Britney’s friend Sam Lutfi exclusively to OK!.

However, Britney might want to get better in a hurry as this third attempt deposing Britney — she has cancelled two previous depositions — was ordered by the court. With that in mind, K-Fed’s attorney Mark Vincent Kaplan tells OK!, "I may exercise my right to ask the court for relief," meaning he might request that the judge take away Britney’s visitation rights.

Spears and Federline have been battling over custody of their two sons, Sean Preston, 2, and Jayden James, 1, for months. This past October, Spears lost temporary custody of her sons and was granted just two daytime visits a week and one overnight visit.

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