Wal-Mart is the latest music vendor to shut down its legacy digital rights management (DRM) system, prompting warnings of potential lost purchases.

The company is advising users to back up any songs which are currently running with the company's DRM software.

When the company shuts down its licensing servers on 9 October, users will be unable to verify new machines or transfer songs to other systems.

Wal-Mart has been selling DRM-free files for more than a year through its online service. The new files are being distributed as clean MP3 files, whereas the DRM-equipped songs and videos were encoded in the WMA format.

"DRM-protected music has been a sensitive issue and we recognise how confusing it can be to customers," lead music buyer Tom Welch said in a blog posting.

"We sincerely apologise for any confusion or frustration our initial email has caused you, and we hope this post and the below points clarify some confusion and alleviate some frustration."

Wal-Mart is hardly the first company to encounter such a problem while transitioning away from DRM-laden systems.

Because the system requires a remote server to authorise machines and devices to play files, users are often left out in the cold when a company dumps a DRM standard and eventually shuts down its systems.

Microsoft faced the same problem earlier this year when it decided to shut down its MSN Music DRM servers and similarly warned users to back up all purchases from the defunct service. The company ended up extending the service for three more years.

A short time later, Yahoo faced a similar situation with its Music Unlimited DRM servers. The company eventually decided to reimburse customers for their purchases.

The Electronic Frontier Foundation (EFF) criticised Microsoft and Yahoo for their initial handling of the situations, and the group had similarly harsh words for Wal-Mart.

"We have warned music fans for years that they could lose their DRM-wrapped music if vendors decided to withdraw support for it. So we are not surprised that three major vendors have done just that," said EFF staff attorney Corynne McSherry.

"What is surprising is that Wal-Mart has not learned from MSN Music and Yahoo Music and made some effort to make things right with its customers."

As in the previous cases, the EFF is asking Wal-Mart to issue an apology to customers, offer refunds, and start a programme to ensure that customers have proof of purchase for their DRM-equipped downloads.