The average downstream speed received by UK households is just 57 per cent of the average advertised rate, according to Ofcom research.
Data from the regulator's hardware-based performance monitoring network shows that while the average broadband package is sold as "up to" 7.1Mbit/s, it actually delivers 4.1Mbit/s.
The UK average is up from 3.6Mbit/s in January however, when Ofcom released its first set of figures.
In rural regions, the gap between marketing and reality is wider, with an average downstream speed of 3.3Mbit/s. In urban areas the mean is 4.6Mbit/s.
As many as one in five customers on "up to" 8Mbit/s packages delivered via the BT network - the most common class of broadband - receive less than 2Mbit/s in reality. Tiscali offers the slowest speeds on average.
Just nine per cent of "up to" 8Mbit/s lines creep above 6Mbit/s.
ISPA, the internet providers' trade association, noted Ofcom's numbers say Lord Carter was working with dodgy data when he made the government's "universal broadband" pledge last month. "The research also shows that the figure in the Digital Britain Report that 11 per cent of UK households are unable to reach speeds of 2Mbit/s is in fact 19 per cent, nearly twice the size and a greater challenge," it said.
The government is pledged to back universal broadband with subsidies. ISPA's "greater challenge" presumably will require greater subsidies.
According to Ofcom, the picture is better for Virgin Media subscribers than for those on ADSL. Those on the "up to" 10Mbit/s package, the cable network's nearest equivalent to "up to" 8Mbit/s ADSL, get about twice the average real world speed (see bootnote).
Market research run alongside performance testing showed that speed problems are the biggest cause of dissatisfaction among broadband subscribers. More than one fifth are unhappy.
The data were collected from 1,600 homes between November 2008 and April 2009. The full report is on Ofcom's site here.
It's part of an ongoing effort to offer consumers more accurate information about broadband, alongside a code of practice calling on providers to be more up front about the limitations of their services. ®
Here's Ofcom's data for the major ISPs it tested. Caution is advised for Plusnet and O2 figures because of small sample sizes. ISPs with less than 2 per cent market share were excluded.
AOL ('up to' 8Mbit/s) - 3.3 to 3.9Mbit/s
BT ('up to' 8Mbit/s) - 3.8 to 4.2Mbit/s
O2 ('up to' 8Mbit/s) - 4.1 to 5.1Mbit/s
Orange ('up to' 8Mbit/s) - 3.8 to 4.5Mbit/s
Plusnet ('up to' 8Mbit/s) - 3.8 to 4.9Mbit/s
Sky ('up to' 8Mbit/s) - 4.0 to 4.7Mbit/s
Talk Talk ('up to' 8Mbit/s) - 3.8 to 4.6Mbit/s
Tiscali ('up to' 8Mbit/s) - 3.2 to 3.7Mbit/s
Virgin Media('up to' 10Mbit/s) - 8.1 to 8.7Mbit/s