RealNetworks has appealed to Apple to join it in forming a common front against Microsoft in the digital music business.

The offer was reportedly made in an email sent on April 9 from RealNetworks CEO Rob Glaser to Apple CEO Steve Jobs.

The New York Times reports that if an alliance with Apple could not be struck, Glaser's email hinted that he might be forced to form a partnership with Microsoft to pursue "very interesting opportunities" because support for Microsoft's media-playing software seems to be growing.

The email message was obtained by The New York Times "from a person close to Apple". According to the Times, Glaser asked Jobs to consider licensing Apple's Fairplay digital rights management system to RealNetworks to permit customers of the RealNetworks music service to play their digital music collections on iPod players.

In exchange, RealNetworks volunteered to make the iPod its primary device for the RealNetworks store and for the RealPlayer software.

According to the Times report: "The message notes that both RealNetworks and Apple support the same digital music technology standard, known as AAC. But because it is not possible for RealNetworks' encrypted music services, Rhapsody and the Real music store, to be played on iPod, RealNetworks is considering switching to Microsoft's competing WMA format, which would make the RealNetworks services work seamlessly with Microsoft's technology."

Glaser wrote: "We are seeing very interesting opportunities to switch to WMA. Instinctively I don't want to do it because I think it leads to all kinds of complexities in terms of giving Microsoft too much long-term market momentum."

Apple executives would not comment on the message. But the Times expects that Jobs will rebuff the offer. Glaser said he had not received a response from Jobs.

In an interview with the Wall Street Journal, Jobs dashed any hopes that Apple would concider allowing other companies to use key Apple technologies. He said: "The iPod already works with the No. 1 music service in the world, and the iTunes Music Store works with the No. 1 digital music player in the world. The No. 2s are so far behind already. Why would we want to work with No. 2?"

A pact with Microsoft would be surprising since it was RealNetworks' complaints about Microsoft's business tactics that formed a major part of the European Commission's antitrust case against the company.