According to the Advertising Standards Agency (ASA), broadband providers are within their right to use the term "up to" if most people can achieve close to the advertised speeds. The Which? survey suggests that this is not, however, the case.
Malcolm Coles, editor of which.co.uk, said: "It's 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 you'll reach the advertised speed it should be made clear up front, so that you know with some certainty what you're buying."
Broadband providers have also been heavily criticised recently for marketing their connections as "unlimited" when in fact there are often daily quotas and speed limits in place.
Orange was the most recent ISP to be rapped by the ASA for failing to mention the monthly download limit stated in its "fair use" policy.
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