The advertorials promoting Interflora ahead of Valentine's Day also resulted in the florist losing search rankings, as described in a blog post published on Friday by SEO expert Anthony Shapley.
Shapley explains that Interflora was "exceptionally aggressive" when preparing for Valentine's Day. He estimates the company placed more than 150 advertorials on UK news sites.
Shapley, who works in the David Naylor SEO team, demonstrates in the post how more than 40 local news sites, plus national titles The Independent and The Scotsman, have experienced a drop in PageRank, falling between four and six points, the majority of them falling to zero.
The regional and national sites had flagged up that the Interflora posts were sponsored content, including the word "ad feature", "advertisment" or "advertorial".
Advice for news sites: Understand links as votes
SEO trainer and journalist Adam Tinworth told Journalism.co.uk the easiest way to make sense of sponsored links is to think about back links as votes.
"Once you start thinking in terms of votes and thinking in terms of buying and selling votes, you can see why there's more of an issue. If people can buy votes and promote a page, it undermines everything Google is trying to achieve."
Tinworth also pointed out that a site's PageRank does not belong to the site, much in the same way that a person's credit rating does no belong to the individual. "And, of course, you cannot sell what does not belong to you," Tinworth added.
What news publishers can sell is their traffic, Tinworth said, offering advertisers the eyeballs of readers. The advice is therefore not to include links in advertorials or to add a 'rel="nofollow"' attribute to the link.
Google's advice for news sites
On Friday Google issued a reminder to news sites about selling advertorials with links.
"Please be wary if someone approaches you and wants to pay you for links or "advertorial" pages on your site that pass PageRank," the post on Google's Webmaster blog states.
Matt Cutts from Google explains that the use of advertorials and pages with embedded links "violates our quality guidelines, and Google does take action on such violations".
The post tells publishers to remove paid links or advertorials, or to add "nofollow" attributes to links.
What about native advertising?
With an increasing number of sites such as The Atlantic, BuzzFeed and Gawker publishing sponsored content, how do they ensure that they retain their search rankings?
Mathew Ingram on paidContent points out that BuzzFeed founder Jonah Peretti tweeted to say all of the paid content that appears on BuzzFeed goes through Google’s DART system. Ingram explains that this is part of its Doubleclick advertising unit and therefore does not pass PageRank.
Update: We initially said that news sites had dropped in search rankings. We have now amended this to clarify that it is a drop in PageRank.
A source in the newspaper industry, who did not want to be named, described the move by Google as "aggressive".
It is also worth reading this post from Search Engine Land on which asks if Google will penalise Chromebooks, Google Analytics, AdWords and Google+ for using advertorials?
Need to learn about search engine optimisation? Adam Tinworth runs courses for journalists on SEO. The next one-day course will take place on 27 June. See this page for details.