An advertising industry watchdog has chastised Comcast again about the MSO’s claims of having the fastest Internet service.

A year ago, PC Magazine ran some tests comparing various ISPs and found that Comcast provided the fastest downstream service on average. Other MSOs, including Cox Communications and Charter Communications, ranked highly in some markets, with Cox called out for having the fastest upstream, on average.

For all that, however, PC Magazine found Verizon FiOS to be the fastest available by far, but it combined FiOS results with Verizon DSL to arrive at an average that dropped Verizon down the list.

Comcast subsequently began running a variety of ads that included the claim “Xfinity is the fastest Internet service provider in the nation, according to PC Mag,” as well as: “It’s official. We’re the fastest.”

Verizon objected, and that’s when the National Advertising Division (NAD) got involved. NAD studied all of the claims involved and announced that Comcast’s claims could be considered true, but only with qualifications that Comcast is not including in its ads.

So NAD is recommending that Comcast discontinue unqualified claims that Xfinity Internet service is the fastest in the nation.

Comcast could communicate that it is one of the fastest ISPs in the nation, according to the 2011 PC Magazine study, or claim that it is the fastest Internet option for most consumers in its service areas where competitor Verizon’s FiOS service is not available, NAD concluded. Comcast should also note that it shares the top spot with Cox and Charter, NAD noted.

NAD is an investigative arm of the advertising industry’s self-regulatory system and is administered by the Council of Better Business Bureaus.

This is the second time NAD has taken Comcast to task for its advertising claims in the space of the last year. NAD previously determined that Comcast cannot, based on its current offerings, make an unqualified claim in national advertising to be faster than the competition. NAD noted that while Comcast is the fastest Internet option for 94 percent of the 52 million households in its competitive footprint, it is not the fastest where FiOS is available.

Comcast, in its advertiser’s statement, said it will “take NAD’s recommendations into account in formulating its future advertising.”