ISPs face changes to the way they advertise broadband services.
From April next year, providers will no longer be able to advertise maximum speeds for net packages unless 10% of customers can actually get them.
The new rules come from the Committees of Advertising Practice (CAP), the body responsible for writing advertising codes.
But Which?, one of the main campaigners for change, said the new rulings do not go far enough.
There has been huge pressure from industry and consumer groups for changes in the way broadband services are marketed with many feeling current campaigns are misleading consumers.
"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 CAP chairman James Best.
"Advertising is only effective if consumers trust the messages they see and hear," he added.
The new rules state that services cannot be advertised with a headline speed, unless 10% of customers can achieve that. It also calls on ISPs to be clearer about the data caps they set for services.
Which did not think the guidelines went far enough.
"Broadband providers have just been given the green light to mislead consumers. The rules say that providers don't have to state what range of speeds most of their customers experience," it said in a statement.
"That means advertising campaigns can now be based on the experience of a privileged few. If just one in 10 customers get access to the top speeds advertised, that's within the guidelines," it said in a statement.
A recent study by Ofcom found that many services marketed as up to 20Mbps actually achieved an average of just 6.8Mbps.
It recommended that ISPs advertise "typical" speeds so that consumers would have a clearer idea of what they were getting.
"We are disappointed that it appears not to be possible to establish a single, clear and consistent 'Typical Speed Range'. Our view is that this is the best way to ensure that consumers are able to compare the wide range of packages that are available," it said in response to the new guidelines.
CAP also looked at the way ISPs advertise services as unlimited even though they impose data thresholds.
It ruled that broadband providers could still impose limits on the amount of data that users could download, but they must explain the thresholds more clearly.
Which? was unimpressed.
"Unlimited should mean unlimited at your normal broadband speed, but internet service providers will be allowed to slow down a supposedly 'unlimited' connection once a customer goes over a certain threshold," it said.
"Ofcom should step in where the advertising regulators have failed, and make sure consumers can't be misled about the broadband service they're paying for," it added.
Virgin, which has campaigned for changes in the way its rival advertise services, said it was pleased with the new codes.
Jon James, executive director of broadband at Virgin Media, said: "This is a much needed and long awaited victory for consumers. The new rules are a big step in the right direction and the greater transparency will ensure people can make more informed choices.
ISPs will no longer be able to hide behind generic terms or catch-all claims which they simply cannot deliver."