The Advertising Standards Authority (ASA) has warned broadband providers it will ban ads that breach its new rules.
Providers are currently allowed to advertise broadband speeds if they are available to ten per cent of their customers.
However, from May 23rd they must ensure speed claims in ads are based on the download speed available to at least half of customers at peak times and described in the ads as "average".
The ASA believes this will help prevent customers from being misled, as it believes many have bought broadband services and not got the speeds they expected.
Speaking to BBC News, an ASA spokesman said broadband providers have been given fair warning about the new rules. As a result, it expects them to bring their ads in line with the guidance.
"If a broadband provider is making speed claims in its ads post-May 23rd, then it will have to hold the evidence to prove the basis on which that claim is made," the ASA official said.