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The song was officially released on the website of New York radio station Z100. It can now be heard on YouTube.
Music industry magazine Billboard says the single is expected to get its first radio play next week. Several other media reports say Spears could have a new album out in early November.
A spokeswoman for Jive Records, Spears's label, was not immediately available for comment.
Spears, 25, is attempting to resurrect her flagging career. The new single could be the first salvo in a media blitz.
Rumours have been rampant that she will perform at the MTV Video Music Awards in Las Vegas on September 9, and for months she has been saying she is at work on a new album.
The singer has become a major subject in the tabloids for public behaviour that has included being photographed with no underwear under her miniskirts, shaving her head bald, attacking paparazzi with an umbrella and performing in local nightclubs, where she was accused of lip-syncing songs.
Earlier this year, she checked into a rehabilitation facility in California. More recently her divorce from second husband Kevin Federline was finalised.
The pair remain embroiled in a bitter custody battle over their two sons.
Back in the early 2000s, Spears was a top-selling artist.
Her records had sold more than 70 million units, and in 2002 she was ranked as the most powerful celebrity in the world by Forbes magazine. © 2007 Australian Broadcasting Corporation