The Help Notes, which have been created following a review of speed claims requested by the Advertising Standards Authority (ASA), state that if web users will be charged or have their broadband service suspended for exceeding a certain limit, then ISPs should not be using the term 'unlimited'. The CAP also said if a traffic management policy used by the ISP affects the user "unduly", the unlimited claim will be classed as misleading.
Furthermore, any maximum speed claims used in broadband advertising must be based on actual experience of users and ISPs must be able to demonstrate these speeds can be achieved by at least 10 percent of its customers.
The guidance, which will come into effect on April 1, 2012, also says ISPs should include additional information in advertising to accompany a maximum speed claim "to ensure the average consumer is not misled" such as, if relevant, highlighting the fact a significant proportion of users may receive a speed that falls considerably short of that maximum advertised.
The guidance will now be used by the ASA Council when considering complaints about speed and usage claims in broadband advertising.
"This new guidance directly responds to consumer concerns by setting an appropriately high bar for advertisers who want to make speed and 'unlimited' claims in ads," said James Best, chairman of the CAP.
"Advertising is only effective if consumers trust the messages they see and hear. This guidance will help deliver that."
Michael Phillips, from comparison site Broadbandchoices.co.uk, said the guidelines were "a positive step towards clamping down on what have to date been theoretical speed claims that are at best optimistic and in most cases completely unrealistic".
"Setting the advertised maximum speed at a figure that at least 10 percent of customers can actually achieve looks good at first glance – but it's cold comfort for the other 90 percent of customers that can't receive that speed," he said.
"It will be interesting to see what providers opt for. Given Ofcom's recent broadband speed report findings, we would expect to see most providers having to advertise their current up to 20/24Mbps speeds as up to 12Mbps – a significant drop to current figures."
ISP Virgin Media agreed saying the guidance was "a big step in the right direction".
"ISPs will no longer be able to hide behind generic terms or catch-all claims which they simply cannot deliver. However there needs to be vigilant scrutiny to ensure this is genuinely applied to all marketing and that the spirit behind this demand for change is upheld, not just the minimum necessary is done to be acceptable," said Jon James, executive director of broadband at Virgin Media
"This new guidance only requires that 10 percent of customers get what is advertised, but Virgin Media is committed to continuing to lead this industry. We only advertise speeds that at least 90 percent of our customers actually receive and will continue publishing our broadband performance every month."