There is a huge gap between advertised broadband speeds and the actual speeds users can achieve, new research from consumer group Which? has shown.
A survey by the organisation found that broadband packages promising speeds of up to 8Mbps (megabits per second) actually achieved far less.
Tests of 300 customers' net connections revealed that the average download speed they were getting was 2.7Mbps.
As a result, Which? has called on regulator Ofcom and Trading Standards to launch a fresh investigation into UK broadband.
The speed tests were prompted by complaints from members of the public, unhappy with the speeds of their broadband connections.
In the last 12 months more internet service providers (ISPs) have offered services, promising speeds of "up to" 8Mbps.
The Advertising Standards Agency (ASA) has investigated several cases of misleading promotions, most recently asking Bulldog to make it clear in its adverts that speed was dependent on how far away from the exchange people lived.
It ruled that broadband providers could use the words "up to" 8Mbps when describing services as long as customers were likely to get close to those speeds.
The average speed achieved in the Which? trials was 2.7Mbps, with the lowest coming in at under 0.09Mbps, barely at dial-up rates, and the maximum only reaching 6.7Mbps.
Smaller providers Global, Waitrose and Zen come out as top performers in the study, with big names AOL, BT and Virgin Media rated below average.
Malcolm Coles, editor of which.co.uk, said: "It is 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 it's unlikely that you'll reach the advertised speed it should be made clear up front, so that you know with some certainty what you're buying."
A spokesman for watchdog body Ofcom said that it was monitoring the situation. "If we get increasing complaints we may look at what more can be done. We are working closely with the ASA and it is very important that consumers know what they are getting and what they are paying for."
According to the Which? survey, done in conjunction with the speed tests, only one in 10 of its members thought that a broadband service advertised as up to 8Mbps would actually deliver the top speed.
A spokesman for BT confirmed that 8Mbps would be a rarity for users. "Virtually no-one will get it. The laws of physics start applying as soon as it leaves the exchange and you would have to live on top of the exchange to get the full 8 megabits."
However BT said it is currently upgrading its exchanges as part of its 21st Century Network (21CN) programme. It will allow for broadband speeds of up to 24Mbps and will start to be made available from early 2008.