Hours after the pop singer’s first posthumous single “This Is It” was released amid great hype on Monday, it emerged that the tune had been recorded 18 years ago by an obscure Puerto Rican singer.

Moreover the co-author of thattune, “My Way” songwriter Paul Anka, threatened to sue Jackson’s estate for proper credit and his share of royalties.

The administrators of theestate quickly acknowledged Anka’s claims and granted him 50% of the copyright, a potentially massive payday for the 68-year-old Canadian crooner. And an equally massive loss for the estate.

“They realize it’s a mistake, they realize it’s my song, they realize it’s my production of his vocal in my studio and I am getting 50% of the whole project, actually, which is fair,” Anka said in a video posted on the TMZ gossip Web site.

The song dates back to 1983, when it was known as “I Never Heard” — a co-write between Jackson and Anka — and intended for inclusion on an Anka album. But the pair fell out, Jackson took the master tapes and Anka got them back.

The song was eventually released in 1991 after Anka placed it with an unknown Latin singer named Sa-Fire.

But “This Is It” had been promoted as a new Jackson recording, one of a multitude of unreleased recordings likely to come out in the next few years.

“The song was picked because the lyrics were appropriate because of the name Michael gave his tour,” said a spokesman for Jackson’s estate. “We are thrilled to present this song in Michael’s voice for the first time, and that Michael’s fans have responded in unprecedented numbers. The song was co-written by the legendary Paul Anka.”

A spokeswoman for Sony Music declined to comment. One of the estate’s two administrators, John McClain, worked with Jackson at the Sony Corp unit. The other executor is music attorney
John Branca.