In what it says is the first comprehensive broadband survey in the UK, the telecoms regulator said that broadband lines advertised as providing speed of up to eight megabits per second (Mbit/s) were connecting users at an average of just 3.6Mbit/s.
Ofcom said that internet service providers (ISPs) must be more open with customers about speed.
"Ofcom’s broadband speeds code of practice requires ISPs to better explain to new customers what speeds they are likely to obtain in practice, and also to tell them what steps they can take to improve their broadband performance," said its report. "Our research findings indicate that there is both consumer demand for such information and room for further improvement in these areas."
Many speed-reducing factors cannot be controlled by ISPs. The most significant is distance from a phone exchange for DSL users. Those who live further from exchanges have slower connections.
Ofcom said, though, that ISPs could do more to connect users at closer to the advertised speeds. It measured the maximum speed reached by each line during a 30 day trial. This gave researchers an idea of what speeds were possible on a user's equipment and in their location.
The study showed that the average speed delivered to users was 15% short of that maximum, which means that traffic is being slowed by 15% because of ISP factors, such as how many users are allocated to fixed network resources.
"[The maximum speed data] can provide insight into how speeds are degraded by ‘contention’, i.e. when speeds decline as a result of multiple users sharing bandwidth within a provider’s network," said the report. "This metric also provides some insight into how the service that consumers receive relates to the service they were sold; under Ofcom’s Broadband Speeds Code of Practice signatories are obliged to advise an estimate of the maximum line speed at the point of sale."
The research found that 26% of users thought that the speed of their connection did not match their expectations when they signed up to a service.
Last year, Ofcom created a Code of Practice for ISPs and most of the industry has agreed to abide by it. To ensure that potential customers are aware of the discrepancy between advertised speeds and what might be achievable in their homes, it demands that ISPs tell customers what the maximum possible speed is likely to be for their particular connection.
A Which? report in 2007 found that there was a "'huge gap" between advertised broadband speeds and those which customers actually received.
Speed was a crucial issue for customers, with 91% of them saying that it was important in making their choice of ISP.