(Jeff Fusco/Associated Press Images for Comcast)

Comcast's claims that it offers the “fastest Internet in America” and the “fastest in-home WiFi” risks misleading consumers and should be stopped, according to an advertising watchdog administered by the Council of Better Business Bureaus.

The ruling Wednesday from the National Advertising Review Board found that Comcast's reliance on Ookla's Speedtest.net data masks important details that undermine the cable giant's marketing.

For example, Ookla's results — which are produced any time a consumer visits its site to run a speed check — find that Comcast's fastest observed download speeds are generally faster than Verizon's, by a difference of 105 Mbps versus 83 Mbps. But that comparison hides the fact that among the same population of Internet subscribers, Verizon's customers had faster upload speeds.

Although many consumers may give priority to download speeds, upload rates are becoming increasingly important in a world of photo- and video-rich social media. Wednesday's ruling comes after Comcast appealed a similar conclusion by a lower panel, the National Advertising Division. NAD had ruled against Comcast as far back as August in response to a complaint by Verizon over the cable company's advertising.

The Speedtest results, NARB said Wednesday, are “not a good fit for an overall claim that [Comcast] delivers 'America's fastest Internet.'" While NARB did not question the accuracy of Ookla's measurements, it noted that the Comcast customers who used Speedtest.net generally were subscribed to faster data plans than the Verizon subscribers, which could have skewed the data. What's more, the watchdog said, it's not clear that the sampled customers were representative of either company's subscriber base.

Comcast's other claims that it offers the “fastest in-home WiFi” could have duped consumers into thinking that by signing up for the company's services they were getting the fastest wireless on-ramp to the Web, according to NARB. In fact, NARB said, that statement largely deals with the speed of customers' home routers and rarely has a significant effect on a household's overall broadband speeds.

NARB urged Comcast to either stop using the claims in connection with Ookla data or to be clear about what the data represents if it continues to use the numbers for marketing purposes. The recommendations are not legally binding. But Comcast agreed Wednesday to abide by the ruling despite its disagreement with NARB's conclusions.

“Comcast will take NAD's recommendations into account in developing future advertisements, and expects NAD and NARB will hold all advertisers to the same standards when making similar claims,” the company said in a statement.

Comcast could still use its “fastest Internet” claims, without using Ookla data to back it up. It could, for example, tie it to its 2 Gbps broadband service, which allows users to download an HD movie in a matter of seconds. But, at a cost of $300 a month, Comcast's Gigabit Pro is largely out of reach for many consumers.