The Committee of Advertising Practice (CAP), which sets advertising guidelines for the Advertising Standards Authority (ASA), said numerical speed claims in broadband ads should be based on the download speed available to at least 50 per cent of customers at peak time and described in ads as "average".
However, from May next year, firms will only be able to advertise the speed that at least half of their customers get.
The body said that consumers may now interpret a range publicised by broadband companies as the speed they are likely to get individually, as opposed to the range that they generally are likely to get.
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.
A recent report from the Post Office revealed that two in five broadband customers suffer from a slow internet connection, 38 per cent have had their connection drop at random times and around a fifth experience a poor Wi-Fi connection in certain rooms of their homes. Ofcom defines peak time as 8pm to 10pm for residential customers.
"Our new standards will give consumers a better understanding of the broadband speeds offered by different providers when deciding to switch providers".
"I'm delighted to see that Cap is finally changing the way broadband speeds are advertised".
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Director of the Committees of Advertising Practice, Shahriar Coupal, said: "There are a lot of factors that affect the broadband speed a customer is going to get in their own home; from technology to geography, to how a household uses broadband".
UK Minister of State for Digital & Culture, Matt Hancock, said the new standards are a victory for consumers.
Alex Neill from consumer group Which? said millions of households were now experiencing broadband speeds that do not meet expectations.
Colum McGuire, senior campaigner at consumer rights group Which? said he hopes today's announcement "will finally ensure people get a clearer idea of the speeds they could actually get in their home before they sign up to a new deal".
Services now marketed at up to 76Mbps are likely to be in the 45 to 55Mbps region, he added, while those advertised as up to 17Mbps could fall as low as 6Mbps under the new rules.
It based its decision on research that found that consumers did not notice fibre claims in adverts, did not treat them as a priority when choosing a package and saw fibre as a "shorthand buzzword" to describe modern, fast broadband.
The change forces internet service providers to more accurately advertise the broadband speed customers will get.
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