Those trying to buy music using versions other than the current iTunes version 4.6 are prompted with a warning and urged to upgrade to the latest iTunes.
"You are using a version of iTunes that will soon no longer be supported," the message says. "Upgrade today to take advantage of the latest application features and music store enhancements."
An Apple representative did not immediately have a comment on the move. The warning comes before a music-related event featuring members of U2 that Apple has scheduled for next week. A new version of iTunes could be on tap when Apple expands its European store to new countries, something it has said it will do this month.
Apple has in the past used various incentives to move users to new versions in an effort to thwart add-on programs that allow music sharing via iTunes. The iTunes program is designed to allow users to listen to, but not download, music stored on closely networked computers. However, there are a growing number of programs that exploit the music-streaming feature and allow the files to be downloaded.
The Cupertino, California, company blocked one generation of those programs with iTunes 4.5, but updated software again allowed consumers to download music that was being shared from another computer.
Apple also faces the threat posed by RealNetworks, which says its Harmony technology allows music purchased from its store to play on iTunes. Apple has warned that it would seek to break that compatibility via a software update to the iPod.
"It is highly likely that Real's Harmony technology will cease to work with current and future iPods," the company said in its July statement.
Apple's warning to iTunes customers was noted earlier this week on Mac enthusiast site AppleInsider.
