UTV has been ordered to scrap an advertisement for its property website which claimed to be "NI's largest".
The Advertising Standards Authority said the ad by PropertyPal.com misled the public.
The complaint against the UTV website was lodged by Propertynews.com - a rival site owned by the Belfast Telegraph from the Independent News & Media Group.
The PropertyPal ad claimed it offered "more choice than anywhere else".
This was misleading, the ASA said.
The claim that its website was the largest was also misleading, the independent watchdog ruled. In all, UTV was told its advertisement broke the advertising code on three counts.
The station argued that it cleared the ad for screening on the basis of information provided to it by the site, which it believed showed that it was the largest property website in Northern Ireland in terms of homes for sales or to rent.
It said it had monitored and kept records monitoring sales to substantiate the claim.
But the ASA said it has broken the advertising code on each of three challenges made by Propertynews.com.
Emma McNally of Propertynews.com said the ruling was "very satisfactory".
Errol Maxwell, managing director of PropertyPal.com said: "We are confident that we are by far the largest property website in Northern Ireland.
"We are currently awaiting the independent verification requested by the ASA to substantiate this position for our future advertising."