Britney Spears‘ first husband is in the slammer.

On Wednesday, Jason Alexander began a 10-day sentence in Los Angeles County jail for a 2006 DUI arrest. The former Mr. Spears ended up behind bars because he failed to complete a court ordered alcohol rehabilitation program.

“Jason messed up and he knows it,” an insider tells Star. “He’s going to keep his head down, stay out of trouble and hopefully he won’t have to serve the entire 10 days.”

Britney married Jason, her childhood friend, at 5:30am on January 3, 2004 at the Little White Wedding Chapel in Las Vegas. The marriage lasted 55 hours ending in an annulment granted by the judge on January 5, 2004.

These days Jason, 27, is living in LA’s San Fernando Valley working for a clothing company.

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