By Mark Savage
Entertainment reporter, BBC News
Legal song downloads now account for 78% of all UK single sales
If you walk into a high street record store today, try conducting a small experiment.
Walk past the racks of DVDs and Justin Timberlake CDs and search for this week's Top 40 singles.
You might find it more difficult than you think.
In Virgin's flagship megastore on Oxford Street, for example, the "singles wall" has been moved to the back of the shop, and only features the Top 20 singles.
Other record stores have stopped stocking the format altogether, with occasional exceptions for big-sellers such as the Arctic Monkeys or Shayne Ward.
The reason is that the singles market has largely moved online - and it has happened almost overnight.
"There's been an absolutely immense change," says Paul Williams, managing editor of music industry trade paper Music Week.
"A couple of years ago there was no such thing as a legitimate digital singles market.
"But now digital makes up something like 80% of the singles market as a whole."
Despite the overwhelming popularity of downloads, not all online purchases are eligible for the Top 40.
Chart rules state that digital sales can only be counted one week before a single is released - and for two weeks after it is removed from sale.
Nelly Furtado would still be in the Top 40 if downloads were counted
"The current chart is actually fairly arbitrary," says Top 40 analyst James Masterton, who writes for Yahoo Music.
"A number of songs that are selling strongly have been removed."
For example, Nelly Furtado's hit song Maneater should still be in the Top 40 this week - but, because the CD is no longer available in shops, the song has been dropped from the chart.
The British Phonographic Industry (BPI) admits that this is "a rather odd scenario".
"Ultimately, the chart has to reflect what people are buying," says spokesman Matt Phillips.
In order to make this happen, the record labels and retailers which control the chart have proposed changing the rules so that digital singles are counted from the minute they go on sale.
The plans are still being discussed, but they are expected to come into effect in January next year.
For music lovers, it could mean they have greater control over the Top 40 - even down to choosing which album tracks get picked as singles.
"For years we've had a situation where the industry decides what the single will be," says Mr Phillips.
"Digital has changed that and it's up to the consumer to choose what songs they like best - and that's a very exciting place to be."
But Mr Masterton doubts that record companies will give up control so easily.
"Rules will be set up to decide when a particular track can be counted as a single and when it can appear in the charts," he says.
"The last thing a record company wants is for an artist to be associated with a particular song and nothing else."
Some analysts believe the new chart regime could be beneficial for smaller labels and new artists.
"Unsigned bands will break top 75, and even the top 40, quite quickly and that will be quite revolutionary," says Ben Drury, whose indiestore website helps artists to sell downloads.
Others are less optimistic about the liberating potential of the new rules.
"The major labels control the market and, whether a single is out on download or on a CD, that will always be a problem," says Martin Goldschmidt, managing director of the Cooking Vinyl label.
It has been suggested that the new rules merely pave the way for record companies to phase out CD singles altogether.
Some independent labels have already taking this step.
Sales of vinyl are on the increase
"For one tenth of the cost, a download single can provide the same effect as a CD single," says Popfiction boss Pete Mash.
"A download can even be quite a personal thing. It comes from the band direct, even though the medium to obtain it is quite cold and mechanical."
But Mr Phillips says singles will stay in the shops for the foreseeable future.
"People like to predict the death of this and that format but I'm not sure we're going to see CD singles disappear overnight.
"Consumers do like physical products. The fastest growing format in terms of sales is seven inch vinyl."