Which? has suggested many households across the country get less than half the speed they pay for, with customers on a 38Mbps service said to receive average speeds of just 19Mbps.
The study - which was taken from 235,000 uses of its speed checker tool - shows that those on super-fast packages offering up to 200Mbps averaged at just 52Mbps, reports the BBC.
The new rulings mean that from May 23, broadband providers won't be able to advertise "up to" speeds if that speed is received by less than 50% of their customers at peak times.
Which?'s managing director of home services, Alex Neill, said: "This change in the rules is good news for customers who have been continuously let down by unrealistic adverts and broadband speeds that won't ever live up to expectations.
"We know that speed and reliability of service really matter to customers.
"And we will be keeping a close eye on providers to make sure they follow these new rules and finally deliver the service that people pay for."
Meanwhile, others - including provider CityFibre - has urged the ASA to go further by distinguishing between fibre and copper connections.
Founder and chief executive Greg Mesch added: "Although we welcome the new rules on advertising speeds coming into force, the ASA hasn't gone far enough to stop consumers from being misled by broadband adverts.
"Fundamentally, the service you get is about more than speed, as capacity and reliability are now as just critical.
"The current rules do not distinguish how fibre and copper-based services are described, despite the experience they deliver being worlds apart."