Current standards allow firms to advertise "up to" speeds as long as they are available to a minimum of just 10 per cent of customers, resulting in widespread complaints from government, consumer groups and the public.
The body says numerical speed claims in broadband ads should be based on the download speed available to at least 50% of customers at peak time and described in ads as "average". Advertising speeds only a fraction of customers can receive helps no one - it damages provider reputation and stokes fury among customers.
ISPA Chair Andrew Glover said: "ISPA supports today's change to rules governing the advertising of broadband speeds as an important way of providing consumers with clear and accurate information".
"Our new standards will give consumers a better understanding of the broadband speeds offered by different providers when deciding to switch providers". "Research commissioned by the ASA persuades us that tougher standards are needed to prevent consumers from being misled by advertised broadband speed claims".
Advertisements, in future must be based on the available speeds to at least half of customers at peak times.
CAP said the median peak-time download speed was the most meaningful measure for consumers at it was easily understood and allowed for comparisons between different ads, while a 24-hour measurement had the potential to mislead by not providing an indication of the speed customers were likely to receive at the time when traffic is heaviest.
UK Minister of State for Digital & Culture, Matt Hancock, said the new standards are a victory for consumers.
Using the term "fibre broadband" in adverts for part-fibre services is not "materially misleading", the Advertising Standards Authority (ASA) has said.
The days of ISPs claiming broadband speeds that ordinary customers could never hope to achieve might finally be coming to an end, after the Committees of Advertising Practice (CAP) issued proposals to crack down on misleading claims.
Alex Neill from consumer group Which? said millions of households were now experiencing broadband speeds that do not meet expectations.
CAP also ruled speed-checking facilities, for example those on ISPs; websites, should be promoted in ads wherever possible.
The new rules are being put into force to tackle this discrepancy.
Richard Neudegg, a regulation expert at uSwitch.com, said: "Currently 90% of broadband customers can be left disappointed by a service that will seemingly fall short of the speed they've seen advertised".
"However, consumers may start to see a much wider variety of speeds in adverts, and with the addition of the peak time period (defined as 8pm to 10pm) there is likely to be more variation between providers. Others may not sell the advertised service but instead push customers to a technically identical service marketed under a different name".