Britney Spears is struggling to keep her young sons in check - the troublesome toddlers won't stop swearing.

The Toxic hitmaker has cleaned up her wild antics following her very public meltdown in 2008 in a bid to win back custody rights of her two sons, Sean Preston, three, and Jayden James, two, from ex-husband Kevin Federline.

But it seems the superstar's maternal mantle is slipping - onlookers were shocked when she failed to discipline the tots when they repeatedly shouted, "Oh s**t," at a celebrity event, according to the New York Daily News.

An insider tells the publication, "She was too busy picking out freebies to chastise the boys for misbehaving.

"We were all surprised by their potty mouths (swearing), but it was actually pretty funny."

Spears has previously blamed the boys' bad language on Federline, insisting they pick up naughty words from their dad.

She said, "(The boys) are starting to learn words like stupid, and Preston says the f-word now sometimes. He doesn't get it from us. He must get it from his daddy. I say it, but not around my kids."