Warner Music has dropped its opposition to selling music online without copy protection.

It means the record label's catalogue will now be available through the US version of Amazon.com's download service.

The move also represents another significant change in the music industry's attitude to music on the internet.

Until now Warner has refused because the songs, in an unprotected MP3 format, can easily be copied and written to CDs. Its change of mind comes after EMI and Universal made similar decisions earlier in the year and leaves only Sony BMG still opting out.

At the beginning of the year Warner chief executive Edgar Bronfman said Apple's proposals to lose copy protection were "without logic or merit".

Now though, the company reportedly believes it has no other option and needs to boost online sales as hard copy revenues fall.

Bronfman announced the move to employees in an e-mail. He explained: "By removing a barrier to the sale and enjoyment of audio downloads, we bring an energy-sapping debate to a close."