Three months after TripAdvisor subsidiary Cruise Critic came under fire for publishing reviews allegedly influenced by Royal Caribbean Cruises Ltd., TripAdvisor itself is facing renewed heat about the integrity of some of its consumer reviews.

Paradoxically, the criticism was triggered by warnings that TripAdvisor displays about the credibility of certain reviews.TripAdvisor posts warning notices alongside displays for the Radisson Hotel Fossil Creek in Fort Worth, Texas, Hotel Renew in Honolulu, the Sheraton Hotel and Sioux Falls Convention Center in Sioux Falls, S.D., the Shelborne Beach Resort in Miami Beach and, reportedly, dozens more.The notices, posted in red type above the flagged property’s TripAdvisor rating, state: "Message from TripAdvisor: TripAdvisor has reasonable cause to believe that either this property or individuals associated with the property may have attempted to manipulate our popularity index by interfering with the unbiased nature of our reviews. Please take this into consideration when researching your travel plans."

Jeff Tucker, a technology consultant and an editor at the Beat of Hawaii blog, kicked off the new round of criticism when he discovered about 90 such messages on TripAdvisor.com, although it turns out TripAdvisor has been posting such warnings since 2006.

"This doesn’t address the likelihood that a huge percentage of all their reviews are fraudulent in one way or another," Tucker wrote. "Perhaps the warning is a start, but I question if they would post one if the hotel was one of their advertisers."

Tucker told Travel Weekly that he values reviews in general but approaches TripAdvisor’s user reviews "with a high degree of skepticism."

That skepticism is growing as a result of his sleuthing, which kicked off a new debate in the blogosphere about the integrity and worth of user-generated content.

Guidebook publisher Arthur Frommer, a longtime critic of travelers’ reviews, questioned whether TripAdvisor "contains within itself the germs of its own undoing" because hoteliers logically would take steps to encourage positive reviews, he argued.

Chris Elliott, who writes for National Geographic Traveler and MSNBC.com, wrote on his blog that he "uses TripAdvisor when I travel, but I do so with the knowledge that the travel industry is successfully manipulating the site."

Robert Cole, a travel industry marketing and technology consultant, said he does find value in TripAdvisor’s hotel reviews, but he added that "the validation aspect has always been a bit tricky with TripAdvisor, and a number of properties and third parties have taken advantage of this to improve their ratings and ranking results."

"It is extremely difficult to make much difference in the rankings [of high-profile hotels and brands] due to the sheer volume of legitimate reviews," Cole said. "The real issue arose with new properties and smaller niche players in major destinations or properties in smaller markets."

Jeff DeKorte, vice president of product for Travel Ad Network, a TripAdvisor competitor, said the controversy about TripAdvisor’s reviews was "inside baseball" as far as advertisers were concerned.

TripAdvisor has a "strong brand and a large audience," DeKorte said, adding that he believed review manipulation had been relatively isolated.

"Advertisers will look for [online media] like that to spend their money," DeKorte said. "Anyone who can reach a large audience of travelers early in the planning process is going to be an appealing place for travel advertisers."

TripAdvisor spokesman Brooke Ferencsik said the site screens every review, has automated tools to blunt attempts to subvert the review system and relies on "more than 25 million monthly visitors to help screen our content."

Ferencsik said that when TripAdvisor uncovers hotel employees writing glowing reviews, trashing competitors, persuading guests to remove negative reviews or providing incentives for customers to write positive reviews, it posts the notices. Moreover, he said, it posts them regardless of whether TripAdvisor and the offending property have an advertising relationship.

"We view our notices and ratings drop as the best punishment rather than dropping [the properties] from the site," he said. "That way, travelers can make the most educated and informed decisions, seeing the good, the bad and the ugly before they book."

In the Cruise Critic controversy, the focus of the criticism was Royal Caribbean, which organized a group of frequent cruise reviewers into the Royal Champions. The line said it didn’t try to unduly influence the group but conducted special events for the group, much as it would for the news media.