Apple has issued a statement in response to RealNetworks' Harmony technology, threatening that company with legal action.
Real stands accused of hacking Apple's technology and may face legal action from the Mac manufacturer under the US Digital Millennium Copyright Act (DMCA), the statement warns.
Apple says: "We are stunned that RealNetworks has adopted the tactics and ethics of a hacker to break into the iPod and we are investigating the implications of its actions under the DMCA and other laws."
Apple also warns users considering using Real's Harmony music service that it may block the hack Real has made.
"We strongly caution Real and their customers that when we update our iPod software from time-to-time it is highly likely that Real's Harmony technology will cease to work with current and future iPods," the company said.
Real claimed earlier this week that its engineers achieved the technology through analysing publicly available information.