Anti-piracy technology was tested on Michael Jackson's new single

Jackson's record label printed copies of the single to distribute to radio stations with key2audio technology that means it will only play in conventional CD players.

The company took the step after copies of the single - released on 7 October - appeared on the internet.

The German 'N Sync release will not play in Windows PC CD players, meaning it cannot be copied on to a blank CD, sent over the internet or ripped into an MP3 format.

A sticker on the case warns it is not playable on computers.

Error message

The New Scientist reports this version will, however, play on Apple Mac terminals.

If listeners try to connect their standard CD player to a CD recorder, the German disc provides error messages, "copy prohibit" or "cannot copy".

The UK and US versions can be played on PCs and burned onto blank CDs but cannot be copied on home CD recorders.

A test to determine the potential to copy using a standard CD player with the US version found that a "no disk" message is displayed.

Sony is not revealing the technology it is using, stating only: "We continue to test available copy protection technologies, and our goal is to implement copy protection on a broader basis to deter digital piracy."

Anti-copying systems are still considered to be in their infancy. They are being developed by companies such as Macrovision and Midbar Technologies.

They have come about because of the success of file-swapping sites such as Napster.

Napster allowed users to take a music file - either taken from a CD or stored on a computer's hard drive - and trade it with an online partner.

To do this, Napster used file-compression technology MP3, which allows music to be sent across the internet in far less time than it would otherwise take.