Napster Inc. said Sunday that it will begin selling music downloads as unprotected MP3 files starting this spring.

The file format change will apply to single tracks and album purchases, while tracks downloaded as part of the company’s monthly music subscription service will continue to have copyright restrictions, the Los Angeles company said in a statement.

Traditionally, protected music files are not compatible with most portable music devices, including Apple Inc.’s iPod, Microsoft Corp.’s Zune and mobile phones that play music.

“The ubiquity and cross-platform compatibility of MP3s should create a more level playing field for music services and hardware providers and result in greater ease of use and broader adoption of digital music,” Napster’s Chief Executive Chris Gorog, said.

The company did not provide specifics as to which labels agreed to license music for sale as MP3s via the Web site.

Napster also recently told subscribers that it will increase its fees on its basic subscription plan from $9.95 to $12.95 a month beginning Jan. 30 but added that it will give existing subscribers the option to lock in the lower monthly fee if they pay for a full year in advance.

Napster reported in November that it had about 750,000 paid subscribers.