Billboard said on Wednesday that the infectiously catchy song was already among the 40 most-played tunes on U.S. radio just two days after being released.

The song went to the top of iTunes in the United States just an hour after it was released on Monday, and has since reached No. 1 in Australia, Canada, Brazil, Finland and 20 other nations.

Citing music industry sources, Billboard said the track - the first single off Swift's fourth album, "Red," due in October - could sell 450,000 to 500,000 downloads by the end of the week.

That could make "We Are Never Ever Getting Back Together" the biggest debut for a digital song by a female artist if Swift surpasses the 448,000 download mark set by Lady Gaga's "Born This Way" single in February 2011.

Swift, 22, has sold 17.8 million albums in the United States since bursting onto the country scene as a teenager in 2006. She went on to earn six Grammys, 10 American Music Awards and more than 13 other country music trophies for hit songs like "You Belong with Me," "Mean" and "Fifteen."

Swift is known for penning songs - some vicious and others dreamy - about past boyfriends, and the new single has sparked fevered speculation among fans and celebrity media over whom Swift is referring to in her new tune.

The singer's past musical targets have included actor Taylor Lautner, and musicians Joe Jonas and John Mayer. "We Are Never Ever Getting Back Together" has few obvious hints, but speculation has fallen on actor Jake Gyllenhaal, whom Swift briefly dated in 2011, and folk-pop singer Mayer, once again.

(Reporting By Jill Serjeant; Editing by Bob Tourtellotte and Steve Orlofsky)

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