The survey looked at 300 customers' supposedly 8Mb connections.
On average, customers were actually getting speeds of just 2.7Mb.
Which? is calling on regulator Ofcom and Trading Standards to investigate UK broadband.
Which? say that the lowest speed achieved in their trials was just 0.09Mb, just a little faster than dial-up.
And right at the other end of the scale, the highest speed was 6.7Mb, still a 1.3Mb short of the speed customers were actually paying for.
In the past year, almost all internet service providers (ISPs) have started offering the UK's fastest 8Mb ADSL broadband packages alongside, or instead of, dial-up services.
"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," said Malcolm Coles, editor of which.co.uk.
"If it is unlikely you'll reach the advertised speed it should be made clear up front, so that you know with some certainty what you're buying."
Really a problem?
However, are broadband speeds really as big a problem as Which? would have us believe?
After all, the consumer group's own results show that speeds fall along a scale: the highest might not reach the advertised speed but it still wouldn't be representative to advertise the lowest.
Factors affecting broadband speed include how close you are to the telephone exchange, how many people are being served by your exchange, the length and gauge of the cabling from the exchange to your house as well as any electromagnetic 'noise' from other appliances in your home.
In other words, they are, by their very nature, hard to predict.
Furthermore, a recent Advertising Standards Authority (ASA) ruling on broadband speed advertising prohibits companies from specifically promising maximum speeds and requires them to bill the products as 'up to 8Mb' instead.
Which? commissioned a survey to accompany their speed tests which revealed that, perhaps because of the ASA rule, only 10% of users actually believed that they would reach the top speeds of their package.
If 90% of people don't expect to get the top speed is it really fair to complain that they're being misled?
Perhaps it does, even by the ASA's own measure.
The advertising ruling stated that they were only permitted to use 'up to' if customers were really likely to reach close to those speeds. That doesn't seem to be the case.
"The Which? survey has opened a real can of worms in terms of how broadband products will be advertised," said a broadband analyst from Choose.
"Clearly, if the average speed customers are getting is almost 4 times less than the advertised speed, even the caveat of 'up to 8Mb' falls short of the law."
"It will be interesting to see how the broadband companies react to this. I suspect that until they are challenged, they'll continue to promise speeds that cannot reasonably be reached," he added.
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