The wax figure of late pop star Michael Jackson is unveiled at Madame Tussauds in Hollywood, California August 27, 2009. REUTERS/Mario Anzuoni

Michael Jackson's new single This Is It was released online and on radio on Monday but fans and music experts quickly spotted striking similarities to an 18-year-old recording.

This Is It was promoted as a new Jackson recording and was released around the world nearly four months after the king of pop died in Los Angeles of a prescription drug overdose at the age of 50.

It will be available to buy as part of a two-disc album that hits the shelves internationally on October 26 and in North America on October 27 to coincide with the global limited release of the Jackson rehearsal footage movie This is It on October 28.

Fans said the This Is It song has the same melody and almost identical lyrics to a little-known 1991 recording by Puerto Rican singer SaFire. A version of the SaFire song, called I Never Heard, was posted on YouTube on Monday.

In Los Angeles, 1960s teen idol and songwriter Paul Anka told celebrity web site that he had written the song with Jackson in 1983, and that Jackson himself had recorded it under the title I Never Heard in the early 1990s.

Sony Music's Columbia/Epic Label Group and executors of Jackson's estate did not immediately return calls for comment.

But quoted executor John Branca as saying, We acknowledge that Michael and Paul wrote this song together.

Anka told the website that those handling Jackson's estate had apologized for ripping off my song.

The This Is It track, which features backing vocals by Jackson's brothers, opens with a soft, soulful introduction and the lines: This is it, here I stand/I'm the light of the world, I feel grand.

Jackson's other executor, John McClain, who is also a co-producer of the This is It album, had said in a statement 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 concert promoter AEG Live and Sony Pictures.

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.

Sony Pictures and Sony Music are units of Sony Corp.