The ad on Sky’s website read: "99.9% Sky network reliability." A further line stated: "Super reliable, totally unlimited Sky Broadband Sky Broadband Unlimited has 99.9% core network reliability".
The small print explained that that the network "delivers 99.9% average uptime to your local exchange".
It continued: "Sky's network is part of your overall connection and other factors affect your overall reliability (eg home wiring, equipment & websites you visit). Sky Network areas only."
However a complaint by Virgin Media questioned whether the ad misled consumers because it implied that the reliability of the complete broadband service was 99.9 per cent.
The adjudication said Virgin Media believes that the core network for Sky "only handled a small proportion of the data flow required to download online content or access a web page".
Sky said it made the small print clear with the use of asterisks, and therefore does not believe that the ad was misleading.
The company said it was willing to clarify the statement that read: "Other factors to affect your overall reliability".
The Advertising Standards Authority agreed that the "99.9% Sky network reliability" headline gave the impression that it referred to the complete broadband service a customer would receive.
It also said that some of the language used in the ad may not have been understood by consumers, for example "average uptime to your local exchange".