By Jim Reed
Newsbeat technology reporter
Online music store 7digital reckons that sales have "rocketed" since it started offering tracks that can be played on any computer or digital music player without built-in copyright protection.
7digital say people are realising that MP3s are more flexible
The company signed a deal with record label Warners in March to sell high quality MP3 files alongside the standard Windows Media Audio (WMA) format.
Since then sales have more than doubled, according to the site, with albums from artists like Madonna, REM and Pendulum all topping the charts.
In June, 7digital signed a second deal with another big record label, EMI, adding MP3 tracks from bands like Coldplay, Goldfrapp, Hot Candy and The Kooks.
The site's boss, Ben Drury, said: "Music fans are beginning to realise that owning an iPod doesn't mean just buying tracks from Apple's iTunes store.
"As the legal digital market has evolved, compatibility has become more of an issue. Customers upgrade and change their devices and want portability of their music libraries."
Switch to DRM-free
Most legal online music stores have traditionally sold tracks in Microsoft's WMA format.
Songs are protected by Digital Rights Management (DRM) technology that is meant to stop illegal copying by restricting both the type and number of machines that tracks can be played on.
iPod users could benefit from cheaper MP3 files
The most popular online music store, Apple's iTunes, encodes files in a slightly different AAC format that can only be played on a compatible computer or Apple iPod digital music player.
But record labels are slowly opening up their back catalogues to websites that sell files without built-in copyright protection.
MP3 files can be copied indefinitely and played on virtually any machine from iPods and Sony Walkmans to mobile phones and the latest stereo systems that accept MP3 memory sticks and CDs.
More set to follow
Companies like 7digital, Tesco and eMusic are already selling thousands of tracks in the format and giant online retailer Amazon is expected to launch its own MP3 store by the end of the year.
Ben Drury at 7digital told Newsbeat: "Record companies have finally realised these locks are ineffective and have made the experience of buying legal music files worse than illegal file sharing. The only way to compete with the free sites is to offer high quality MP3 files."
The two other "big four" record labels, Universal and Sony BMG, already sell tracks in MP3 format in the United States with a UK launch "only a matter of time", according to Ben Drury.
"They really need to do it before the start of the key Christmas period," he said.
"I would expect an announcement by the end of the summer or early autumn at the latest."