Music company EMI Group said it is suing a US-based website that is selling downloads of Beatles songs.
EMI said it had filed a copyright infringement suit against Bluebeat.com in a US court.
Bluebeat.com's site lists Beatles material for sale, including the Fab Four's re-mastered albums, released in September.
EMI said it had not authorised content to be sold on BlueBeat.com.
Beatles holding company Apple Corps has so far declined to allow the Fab Four's music on any internet music service, including iTunes.
Long-running negotiations between EMI and Apple Corps over making the band's catalogue available online have stalled, although EMI said the talks are continuing.
BlueBeat is owned by Santa Cruz, California-based Media Rights Technologies. The company could not immediately be reached for comment.