More than a million downloads have been sold via sites such as Freeserve, MSN and Mycokemusic from January-March - 10 times up on the same period in 2003, according to British BBC.
OD2, the firm that provides the downloads, said OutKast's Hey Ya! was the most popular song in the UK. It said it proved there was demand for a "decent, legal alternative to poor quality illegal peer-to-peer networks".
Music fans are downloading a wide range of songs, with the top 100 downloads accounting for just 11% of sales, the company said.
This contrasts with CD single sales, where the top 100 CD singles account for 77% of total CD singles sales.
Meanwhile, a report by researchers at Harvard and North Carolina Universities has suggested that swapping songs online has had no negative effect on music sales.
The research, conducted over 17 weeks in 2002, blamed "a reduction in music variety" and "a consumer backlash" for declining sales.
But official music industry bodies have branded the study "skewed".
The study says illicit song-swapping accounts for a "tiny fraction" of a 20% dip in music sales over the last three years.
It also suggests that album purchases experienced a peak in the 1990s as music fans replaced their vinyl collections with CDs.