Many broadband customers are not getting the service and connection speed they paid for, it has been claimed.
According to research from Which? Online, over a third of its members have broadband packages which purportedly offer speeds of up to eight megabytes per second (Mbps).
However, it was found that the mean speed of the 300 customers whose connections were tested stood at around 2.7 Mbps, and was sometimes as low as 0.009Mbps.
In response to the survey, Which? has called for the Advertising Standards Authority (ASA) to investigate claims made in broadband providers' advertising campaigns.
Which? Online Editor Malcolm Coles said: "Its shocking that internet service providers can advertise ever-increasing speeds that seem to bear little resemblance to what most people can achieve in reality.
"If its unlikely youll reach the advertised speed it should be made clear up front, so that you know with some certainty what youre buying.
"Do your research to check what speed youre likely to get before upgrading, and if you think what youre getting differs vastly from what youve paid for, speak to your provider - or if they wont help, report them to Ofcom."