By Rory Cellan Jones
Technology correspondent, BBC News
The UK's largest broadband supplier has called for the industry to be clearer about how it advertises net speeds.
What broadband speed are you clocking up?
BT Wholesale, which supplies eight million people, said many customers were disappointed by the mismatch between advertised and actual speeds.
An independent survey found that 15% of people who bought eight megabit per second packages actually got the speed.
The firm said regulators needed to agree rules about how broadband speeds could be sold to the public.
"The reality is we are all trying to push the technology," Guy Bradshaw of BT Wholesale told BBC News.
"The industry needs to join together with Ofcom to agree a set of principles as to how these messages should be communicated and advertised so that the understanding with the consumer is as accurate as it can be."
BT said that, while its DSL Max product offers a range of speeds up to eight megabits per second (Mbps), it tells its customers - the Internet Service Providers (ISPs) - that actual speeds will vary from user to user.
Cameron Rejali, Managing Director of Products at BT Wholesale, said it is up to the ISPs how they market broadband, "but if they are marketing it badly, the market will punish them."
BT said users need to know that there is a difference between the line speed - what the line between their home and the exchange can support - and what it describes as "throughput", a measure of the data coming down the line during an activity such as the downloading of a video.
Only 35% of BT's DSL Max customers are achieving an eight mbps line speed - the rest will see their speed cut by factors such as distance from the exchange, poor equipment, and interference from electrical appliances.
But none of these five million users will achieve eight mbps "throughput" because of internet congestion and other network issues.
"The reality is if you are very far from an exchange or there are environmental factors then your speed will come down and there is not much we can do in the short-term to address that problem," said Mr Bradshaw.
Ofcom is currently reviewing the way broadband is marketed to consumers.
A spokeswoman said: "Whilst there are technical reasons why a consumer may not get the full speed of the package to which they have signed up the key point is that consumers should be able to make an informed decision about what broadband package is best for them at the point of purchase."