A lot of consumers receive broadband speeds lower than what companies claim they will get in their advertisements.
The change follows research into consumers' understanding of broadband speed claims.
From May 23rd 2018, adverts must be based on what is available to at least 50 per cent of customers at peak times.
As well as insisting ISPs use "average" instead of "up to" speeds, Cap also urged ISPs to promote speed-checking facilities in their adverts so that users could test out the speeds they were likely to get from any given service.
Currently ISPs are allowed to use headline speeds that only 10 per cent of customers receive. As peak time is when traffic volumes are highest and traffic management policies are most likely to apply, a peak-time measure provides a better indication of the actual speeds consumers are likely to experience.
"To give some idea of the scale of the changes for the partial fibre (FTTC) services now sold as up to 38Mbps, we are expecting to see adverts featuring speeds in the 24 to 30Mbps region then up to 76Mbps services should be in the 45 to 55Mbps region", said Ferguson.
The change comes after research showed consumers may be "materially misled" by the advertising of speed claims based on current guidance.
The new rules are being put into force to tackle this discrepancy.
The ASA has recently carried out a review into whether the term "fibre" should only be used to describe full-fibre broadband services, rather than part-fibre connections that still use copper cables.
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". 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 from consumer group Which? said millions of households were now experiencing broadband speeds that do not meet expectations.
"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".
"People shouldn't expect adverts to change overnight, as most changes are likely to emerge in April just ahead of the deadline", he told the BBC. ISPs were previously forced to combine line rental costs and broadband costs into a singular amount so that people could see what they were spending more easily, especially useful when using price comparison sites as it more quickly enabled you to see what was what.