Plusnet has been forced to pull two radio ads after a complaint was upheld by the Advertising Standards Authority (ASA).
The complainant argued that the terms and conditions in separate ads for a broadband package offer and a mobile tariff deal were spoken too quickly.
This, they stated, meant the ads were "unintelligible" and therefore potentially misleading.
Plusnet responded by insisting that the terms and conditions were spoken at an acceptable speed, with any qualifications or limitations clearly set out.
However, the company said it had recently reviewed its radio ads and was in the process of revising them to make sure crucial information is read at a slower speed and more clearly.
Nevertheless, the ASA upheld the original complaint, agreeing that the terms and conditions were "difficult to make out and that they had been delivered too quickly to be understood by listeners".
"Because those conditions had not been presented in a clear and intelligible manner, we concluded that the ad was misleading," the watchdog said.
Plusnet has therefore been told the ads cannot appear again in their current forms, while the company must ensure "significant limitations and material information" are presented in a clear and intelligible manner in future ads.