America's media watchdog last night promised an investigation after pop star Janet Jackson's breast was exposed during half-time entertainment at the Super Bowl in Houston, Texas.

During the break of the country's premier American football game, which draws one of the largest television audiences of the year, the singer Justin Timberlake was performing a flirtatious duet with Jackson when he tore off part of her black leather bustier to the words "I'm gonna have you naked by the end of this song".

Her breast was partially obscured by a sun-shaped, metal nipple decoration and the network CBS, which was filming the event, quickly cut away from the shot.

None the less, the Federal Communications Commission (FCC) was inundated with complaints about the incident and about some male singers grabbing their groin.

Timberlake blamed a "wardrobe malfunction", but the FCC chairman, Michael Powell, branded it "a classless, crass and deplorable stunt".

"I am outraged at what I saw during the half-time show of the Super Bowl. Like millions of Americans, my family and I gathered around the television for a celebration ... Our nation's children, parents and citizens deserve better," Mr Powell said.

Television networks are already on the defensive, with the FCC taking a more aggressive stand against indecency and Congress threatening to sharply raise fines for such incidents.

Federal rules bar the broadcast of obscene material and limit the airing of indecent material that contains sexual references in a "patently offensive manner" to late-night hours.

Mr Powell promised a "thorough and swift" investigation by the commission. The FCC could fine each station that aired the show as much as $27,500 (£15,000), as well as hold hearings on whether to revoke stations' broadcast licences. If applied to each of CBS's local stations, the fine could run into millions.

CBS apologised for the "unscripted moment", while the National Football League said it was not likely to use MTV, which produced the entertainment, again.

"We attended all rehearsals throughout the week and there was no indication that any such thing would happen," CBS said.