Internet service providers (ISPs) have been warned over misleading customers with their broadband speed claims.

New guidance from the Committees of Advertising Practice (CAP), the bodies responsible for writing the Advertising Codes, states that maximum speed claims should be based on the actual experience of users.

Furthermore, providers should not describe their services as ‘unlimited’ if they charge customers or suspend services for those going over a certain threshold.

The guidance comes as research by telecoms regulator Ofcom showed that average UK broadband speeds have risen in the last year at a rate that has still left many consumers struggling to keep up.

In the year to November 2011, average download throughputs rose from 6.2Mit/s to 6.8Mbit/s, which sounds like a healthy 10 percent increase rate until the wide disparities in performance are taken into account.

Superfast broadband is now available to around half the UK population, with consumers able to get between 17Mbit/s and nearly 50Mbit/s on some of these services, comparable to the best urban broadband connections anywhere in the world.

The problem is that more than 75 percent of UK connections remains stuck on copper ADSL rather than fibre, which makes the speeds they can deliver dependant on fixed influences such as the quality of the line and the distance a residence is from the local exchange.

A consumer group said that CAP's guidance does not go far enough, however, as it allows advertisers to continue to make maximum speed claims if they can demonstrate that at least 10 percent of customers can achieve the speed.

It has already told ISPs to stop using ‘up to’ speed claims in their adverts.

“Consumers are still unable to have an informed choice of which ISP gives them the best internet speeds if only 10 percent of a provider’s customers get the maximum advertised speed,” the Communications Consumer Panel said.

The guidance, which comes into effect on 1 April 2012, was created after the Advertising Standards Authority (ASA) asked CAP to review advertising claims in the telecommunications sector following consumer complaints.

“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 CAP.