It was billed as a brand new track from beyond the grave - and the first song from a treasure trove of unreleased material left behind by Michael Jackson before the pop icon’s untimely death from drug overdose.

But as with so many things involving the late singer, the wildly hyped release of This Is It quickly descended into farce yesterday, amid claims that the tune was stolen from the My Way songwriter Paul Anka, and, worse, that it had already been released 18 years earlier by an obscure Puerto Rican singer named Sa-Fire.

With Anka threatening to sue Jackson’s estate, the administrators quickly granted him a credit in the liner notes and 50 per cent of the copyright, a move that could make the Canadian songwriter millions of dollars.

“They realise it’s a mistake, they realise it’s my song, and they realise it’s my production of his [Michael Jackson’s] vocal in my studio, and I am getting 50 per cent of the whole project, actually, which is fair,” Anka said in a video posted on the TMZ entertainment website yesterday.

In fact, it emerged that This Is It was originally written by both Anka and Jackson in 1983, when it was then known as I Never Heard.

The song was supposed to be included on one of the Canadian artist’s own albums, but two men reportedly fell out, and Jackson took the master tapes, although Anka eventually got them back and gave the tune to Sa-Fire, who released it as I Never Heard in 1991.

While the melody has remained unchanged over the years, Sa-Fire’s version of the song, an up-tempo dance-pop number with dated 1980s production gimmicks, is dramatically different from This Is It, which has been slowed down to become a rousing R&B power ballad, complete with harmonies and finger snaps by the late singer’s brothers.

“The song was picked because the lyrics were appropriate because of the name Michael gave his tour,” said a spokesman for Jackson’s estate, who glossed over the dispute with Anka and the revelation that the same tune had been released 18 years ago.

“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.”

Although it was being streamed free of charge on Jackson’s website yesterday, fans will not be able to buy This Is It until Jackson’s posthumous two-disc album is released in a fortnight’s time, to coincide with the October 28 worldwide release of a movie of the same name.

The movie will feature video footage of Jackson rehearsing for his comeback tour in London in the weeks before he was allegedly administered a fatal dose of hospital anesthetic by his $150,000-a-month personal doctor, Conrad Murray. The death has since been ruled a homicide, and charges are ultimately expected to be filed against Dr Murray.

The movie is the result of a $60 million deal between Jackson’s estate, the privately-owned concert promoter AEG Live, and Sony Pictures. It is expected to help pay off the late singer’s debts and boost the value of his vast music catalogue, thought to be worth about $400 million.

Meanwhile, Jackson’s two-disc album will be a mixture of greatest hits, two versions of This Is It, and several unreleased versions of classic tracks, plus a spoken word poem read by Jackson and entitled Planet Earth.

One of the two administrators of Jackson’s estate, John McClain, this week said that This Is It shows "once again, what the world already knows, that Michael is one of God’s greatest gifts”.

Not all critics agreed. While many greeted the song with enthusiasm, Jon Pareles, the chief pop critic of the New York Times, wrote in a blog posting it “won’t be on anyone’s list of best Michael Jackson songs, even if it’s a long list” and added that he hoped there was something better locked away in the vaults of Jackson’s out-takes.

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