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 that tune, “My Way” songwriter Paul Anka, threatened to sue Jackson’s estate for proper credit and his share of royalties.

The administrators of the estate quickly acknowledged Anka’s claims and granted him 50 percent 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 percent 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.

Both “I Never Heard” and “This Is It” share the same vocal and piano line, although the latter track boasts new overdubs from Jackson’s brothers.

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.

It was released online around the world nearly four months after the singer died in Los Angeles of a prescription drug overdose at the age of 50.

Fans will be able to buy it when a two-disc album hits the shelves in two weeks to coincide with the October 28 worldwide release of the Jackson rehearsal-footage movie “This is It.”

Lyrics fit the bill
“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.

McClain, who is also a co-producer of the “This is It” album, had said in a statement earlier on Monday that the song “only defines, once again, what the world already knows — that Michael is one of God’s greatest gifts.”

Some critics begged to differ. Jon Pareles, the chief pop critic of The New York Times, said in a blog it “won’t be on anyone’s list of best Michael Jackson songs, even if it’s a long list” and hoped there was something better in the Michael Jackson vaults of album outtakes.

The “This Is It” movie is based on rehearsal video shot in Los Angeles in the weeks before Jackson’s planned 50 comeback concerts in London. It was the subject of a $60 million deal between Jackson’s estate and closely held concert promoter AEG Live and Sony’s Sony Pictures unit.

Sales of Jackson’s records spiked after his death and the release of the movie and album will add to the value of the “Thriller” singer’s estate, estimated at around $400 million.

Sony Music said the first disc of the album will feature some of Jackson’s greatest hits plus two versions of the “new” single.

The second disc will include unreleased versions of some of the singer’s classic tracks and a spoken word poem entitled “Planet Earth” performed by Jackson and never heard before.