The charts are currently based on the sale of singles, in physical and download format, but with the rapid rise in the popularity of streaming services, chart bosses have said they are "bound to" include these at some point.
We7 and Spotify, which lets users listen to music online for free, with advertising funding the cost of listening, have soared in popularity.
Spotify, which also offers streaming of songs without ads for £9.99 a month, launched to the public in February and by April had already clocked up more than 1m registered users in the UK.
About 40,000 new users are signing up to the service every day, of which 50% to 60% are from the UK.
However, because listeners do not pay per track, it is unclear how much weight a stream would have compared to a purchase.
Martin Talbot, the Official Charts Company managing director, told BBC News: "The charts have always been there as a popularity poll, as a means of identifying what are the hottest records of the moment.
"That's been relatively simple when people have bought stuff to keep forever. But that's going to become increasingly more complicated.
"Knowing what a stream is worth compared to a purchase of a download, for instance, is very difficult to identify at the moment, but that's obviously going to be the next step."
Talbot said that streams and subscription downloads would be integrated into the main chart when they become "a very big part of the way people consume music going forward", although he did not know if this would happen in five years, 10 years or 20 years' time.
Music downloads started counting towards the Top 40 in 2005.
.jpg)
.jpg)