7digital will now become the sole supplier of HMV's digital operations
Music, games and books retailer HMV says it has bought half of online music store 7digital.
HMV is buying a 50% stake from the venture capital firms that backed 7digital for £7.7m in cash.
7digital will now become HMV's supplier for all of its digital operations, including a new e-books website for its Waterstone's brand.
HMV also said like-for-like sales in the five countries it operates in fell 1.8% for the 18 weeks to 29 August.
Like-for-like sales - which strip out the effects of store openings and closures - in the UK and the Irish Republic rose 1.7%.
HMV's move comes as legal digital sales increasingly count for more of music revenues to major media companies, which have decreased partly due to piracy on the internet.
"With consumer adoption of legal digital media now reaching the mass market, this joint venture partnership is great news for our customers and suppliers," said Ben Drury, founder and head of 7digital.
Music streaming service Spotify has a deal to sell downloads of its tracks on 7digital, whose other clients include Sony, Warner Music, EMI, Universal and Last.fm.
HMV said 7digital planned to launch a mobile application for BlackBerry mobiles soon.
7digital is forecast to make an pre-tax loss of about £1m for this year, HMV said.
The HMV group has 379 HMV stores and 313 Waterstone's stores, and recently established a partnership with Curzon Artificial Eye to trial digital cinemas based in HMV stores.