Home broadband providers must now ensure that at least 50% of their customers can achieve advertised speeds at peak time under a crackdown to prevent misleading claims.
Until today, firms had been allowed to advertise "up to" speeds as long as they were available to a minimum of just 10% of customers, resulting in widespread complaints from Government, consumer groups and the public.
The Advertising Standards Authority (ASA) will begin to enforce the tougher standard - that speed claims should available to at least half of customers between 8pm and 10pm and described in ads as "average" - from today.
The ASA said the new standard would help consumers better understand what is on offer when deciding to switch providers.
The Committees of Advertising Practice (CAP) toughened up the standards following research which found they were likely to mislead consumers.
The study, commissioned by the Advertising Standards Authority (ASA), found most consumers thought they were likely to receive a speed at or close to a provider's headline claim when, for many, that was not the case.
Previous independent testing by consumer groups found that up to three quarters of households were paying for advertised broadband speeds they never received.
ASA chief executive Guy Parker said: "From today, consumers will see a difference in broadband ads that make claims about speed as this new, tougher, standard is enforced.
"We'll be making sure consumers aren't misled by speed claims in ads, not least because choosing the right broadband deal has become such an important part of running a household or business."
Which? managing director of home services Alex Neill said: "These changes will mean that broadband providers will no longer be able to entice customers with unrealistic adverts promising speeds that most of their customers may never get.
"We know that fast and reliable broadband is what really matters to broadband customers and have been campaigning for these changes. We will now be watching closely to make sure providers are finally living up to their promises."
Ofcom consumer group director Lindsey Fussell said: "These new rules will make a real difference in closing the gap between what broadband shoppers expect and what they actually receive.
"We're also making sure broadband providers give people better information upfront, as well as giving customers the right to leave their contract penalty free if their provider fails to deliver the speeds they promised."