The "Gimme More" singer - who scooped three prizes at the recent MTV Video Music Awards - had been putting in "every hour under the sun" in preparation for her comeback in November with new track "Womaniser."

But "hundreds of thousands of fans" have had a preview of the song after it was posted on video sharing site YouTube, along with footage of her rehearsing in a Los Angeles studio.

A source said: "Record company bosses have been frantically trying to pull the material from the internet. It's not good for Britney's comeback if people aren't getting to listen to the polished version."

Meanwhile, friends of Britney fear the build up to the release of new album 'Circus' is taking its toll on her.

The 26-year-old singer - who suffered a public breakdown earlier this year - is said to have lost a "dangerous amount" of weight because of work-related stress.

One pal is quoted by Britain's Daily Star newspaper as saying: "Britney has been putting in every hour under the sun to get herself ready in time for the "Circus" release. It's too much too soon.

This will set her back mentally and physically unless she takes the pressure off. "She's lost so much weight, some feel it's a dangerous amount. Britney is not ready for this."

Record label bosses decided to bring forward the release date of the album to December after her successful appearance at the VMAs.