A CD by Celine Dion is protected with the anti-piracy system

Revealed in late October by Windows expert Mark Russinovich, the software copy protection system hides using virus-like techniques.

One class-action lawsuit has already been filed in California and another is expected in New York.

Digital rights group, the Electronic Frontier Foundation (EFF), is also gathering information from users to see if a case can be brought.

Court claim

The row erupted following Mark Russinovich's discovery that Sony BMG in America was using a so-called "root kit" to conceal the program used to stop some of its CDs being copied.

"Root kits" are being increasingly used by virus makers to hide their malicious wares deep inside the Windows operating system.

Sony BMG used a program called XCP created by UK firm First 4 Internet that employed similar cloaking systems to hide the proprietary media player used to play tracks on 20 CDs made by the music giant and sold in the US.

But since Dr Russinovich wrote about his discovery the row has snowballed and now has led to lawsuits being filed against Sony BMG.

XCP PROTECTED CDS Trey Anastasio - Shine Celine Dion - On ne Change Pas Neil Diamond - 12 Songs Our Lady Peace - Healthy in Paranoid Times Chris Botti - To Love Again Van Zant - Get Right with the Man Switchfoot - Nothing is Sound The Coral - The Invisible Invasion Acceptance - Phantoms Susie Suh - Susie Suh Amerie - Touch Life of Agony - Broken Valley Horace Silver Quintet - Silver's Blue Gerry Mulligan - Jeru Dexter Gordon - Manhattan Symphonie The Bad Plus - Suspicious Activity The Dead 60s - The Dead 60s Dion - The Essential Dion Natasha Bedingfield - Unwritten Ricky Martin - Life

One filed in Los Angeles by Californian attorney Alan Himmelfarb wants to stop Sony BMG selling more CDs protected by anti-copying software and seeks damages for Californians that have bought any albums protected this way.

According to a report in the Washington Post the lawsuit alleges that Sony BMG has broken three Californian laws. At the same time New York lawyer Scott Kamber is planning a class-action lawsuit for all Americans affected.

The EFF is also gathering stories from buyers of Sony BMG CDs protected with XCP. In a statement the organisation said: "We're considering whether the effect on the public, or on EFF members, is sufficiently serious to merit a lawsuit".

At the same time the Italian digital rights group, Electronic Frontiers Italy, has asked the nation's government to investigate Sony over its use of anti-piracy software.

A weblog documenting the unfolding controversy and calling for a boycott of Sony products has also been created.

When contacted a representative for Sony BMG in the UK referred all calls to its corporate headquarters in New York. A call to a spokesman in that office has yet to be returned.

Artist list

The EFF also released a partial list of all the CDs protected with XCP. The list includes popular artists such as Natasha Bedingfield, Celine Dion and Amerie. It also gave advice for ways to spot if a CD is XCP protected.

So far Sony BMG has not released a list of how many CDs are protected or how many have been sold. It has only said that "about 20" titles are protected with the controversial program.

However, the row does not appear to be denting interest in one of the CDs protected by XCP because at the time of writing Neil Diamond's 12 Songs album was the top seller on the Amazon.com website.

Anti-virus companies are starting to release software that can spot the XCP files. Symantec said it had made tools that can find the files but will not remove them.

Computer Associates said that it would be releasing a tool to completely uninstall the XCP program.

At the same time anti-virus firm Kaspersky Labs branded the XCP program spyware because it hides itself, could compromise security and can slow machines down.

Dr Russinovich has continued his investigation of the XCP software and has confirmed that when installed it can make a Windows computer more unreliable.