Britney Spears is stunned her latest song "Do Somethin'" has become an instant US hit - even before being officially released.

The "Toxic" beauty's ballad entered the Billboard chart at number 100 this week despite the fact her label Jive Records has not officially released or promoted the song, reports Ireland Online.

The 23-year-old has her fans to thank for the unexpected hit who, after hearing the single on last year's "Greatest Hits: My Prerogative" album, swamped radio stations with so many requests for it that the song made the chart based on airplay alone.