By Ian Youngs
BBC News entertainment reporter
US hip-hop duo Gnarls Barkley have become the first act to score a UK number one single on the strength of digital sales alone.
The chart landmark comes less than a year after download sales were first counted towards the top 40.
Gnarls Barkley went to number one with 31,000 download sales
Before that, the singles scene looked rather gloomy - sales had dropped from 80 million in the late 1990s to little more than 20 million in 2005.
But then came the legal download services, led by Apple's iTunes, which launched in the UK in June 2004.
They were an instant hit and proved that fans were still interested in buying single tracks - if it was quick, easy and at the right price.
So instead of going to a record shop to hand over £2-4 for their favourite song, fans could instead pay 79p from the comfort of their own homes.
"Downloads have certainly given the top 40 a shot in the arm," says Omar Maskatiya, director of the Official UK Charts Company (OCC).
"In the period between the start of the decline in sales of physical formats and the introduction of legal download services, the relevance of the top 40 was called into question.
"However, that didn't spell an end to the desire for singles by music fans - it just reflected a shift in the way many of us choose to purchase our favourite new tracks.
"The singles market - with download sales now included - has topped the one million mark each week for the last 11 weeks, its highest weekly levels for some years.
"By incorporating the vast majority of legal downloads in the UK the chart continues to reflect the UK's favourite songs."
More than 26 million songs were downloaded legally in the UK in 2005 - up from virtually zero two years earlier.
And downloads now account for about three quarters of all singles sold.
But because many downloads are back catalogue tracks, the high street still accounts for 55% of top 40 sales - a slender lead that is being slowly eroded.
When downloads are confirmed as the dominant force in the singles chart, the music business is likely to face a further dilemma.
Downloads vs shops
On the one hand, the industry needs the chart to reflect the most popular songs of the day - including downloads.
But record shops have so far tried to make sure they do not have too many gaps in their singles racks if songs are only available to download.
A recent rule change means digital sales start counting towards the chart the week before the CD release.
But Gnarls Barkley's song was first made available to download on 13 March, meaning there were two extra weeks when thousands of fans were legitimately buying the song - but it was not appearing in the chart.
Music impresario Tony Wilson says the industry has been too slow to deal with downloads, but says: "I'm just pleased that the digital thing has finally come of age."
Mr Wilson, who was behind bands such as Joy Division and the Happy Mondays, launched one of the first legal download services, Music 33, in 2000.
"The single has never gone away - it's just that the industry lost its way with regards to technology," he says.
It was "kind of obvious" that the internet was going to be the future of music almost 10 years ago, he says.
"Everyone's habits are changing, we're all buying online and me personally, I buy singles online. If I like the band, I go and buy the physical CD album."
Mobile phones could be at the centre of the next big music boom
Music downloads to mobile phones already account for 7% of all chart sales - and that is likely to be the next big boom, Mr Wilson says.
The OCC's Mr Maskatiya says: "We have begun to see increasing activity in the mobile sector and this has been reflected in the volumes that we now see going through those services.
"Online and mobile are different experiences for consumers and are both in continual development.
"But it seems reasonable to expect a greater influence from the mobile sector driven by the rapid introduction of new technology and handsets."
The OCC is also considering including video downloads to the charts, he adds.
So where does all this leave the humble CD single?
Mr Maskatiya says physical sales are now "bottoming out", and there will always be those who cannot, or will not, make the switch to digital songs.
But Gnarls Barkley's hit confirms that downloads are now the most popular and important singles format.
And that is unlikely to change - until the next revolution.