In a joint statement, the companies said the watchdog has "failed in its duty to protect what is undeniably in consumers' best interests - the transparency around the services they are being sold by technology providers".
Previously broadband providers could advertise "up to" speeds as long as they were available to at least 10% of customers.
Today's decision, which will restrict advertising to speeds available to 50% of the customer base, will come into force on 23 May 2018 after a sixth-month implementation period. CAP then consulted with Ofcom, ISPs, and consumer groups to help find a better way for providers to advertise their broadband speeds.
A ruling by the Committees of Advertising Practice (CAP) has said that ISPs will have to alter their advertising to more realistically depict speeds that customers should expect.
Responding to the news, Digital Minister Matt Hancock said: "The clarification on the way fibre services should be advertised is a welcome step in the right direction, and I hope the ASA will keep the matter under review".
Additionally, it was felt that using peak time as a measurement, rather than 24 hours, would give people a clearer picture of what speeds they were likely to receive when they were most likely to be using the internet.
The Committees of Advertising Practice (CAP) is toughening up the standards following research which found they are now likely to mislead consumers.
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However, it advises advertisers to not state or imply a service is the most technologically advanced on the market if it's a part-fibre service.
"While this change might reduce the number of consumers that feel let down, the reality is that a national advert can never accurately communicate broadband speeds because speeds are so specific to your individual property".
When questioned consumers said they didn't notice "fibre" claims in ads and when probed said they took it to mean that the service offered was modern fast broadband.
Alex Neill, managing director of home products and services at Which?, said: "Millions of households are now experiencing broadband speeds that just don't live up to their expectations and unrealistic adverts showing speeds you're never likely to get don't help, The Guardian reported".
CAP also ruled speed-checking facilities, for example those on ISPs; websites, should be promoted in ads wherever possible.
"Which? has been campaigning for action on this, so it is good to see people may finally see the speeds they could achieve before they sign up to a deal".
However, he warned, this may mean that some ISPs might try to wiggle out of providing a service to customers likely to get speeds at the lower end of the scale, or try to push customers to a technically identical service marketed under a different name as a means to get around the changes.