Google, Bing and Yahoo warned over search result advertising
The US Federal Trade Commission has told two dozen search engines to clearly distinguish advertisements in search results to avoid deceiving consumers.
Bing was one of the search engine companies sent a letter about advertising in search results.
The US authority warned that there has been a "decline in compliance" over 2002 guidelines demanding that search engines should clearly separate paid advertising content from "natural" search results.
In the FTC letter, sent this week to seven general search engines and 17 specialist sites, companies were warned that the distinction between paid content and search results had become less noticeable.
The Associate Director for Advertising Practices, Mary Engle, wrote: "Consumers ordinarily expect that natural search results are included and ranked based on relevance to a search query, not based on payment from a third party."
She added: "To avoid the potential for deception, consumers should be able to easily distinguish a natural search result from advertising that a search engine delivers."
The heavily trafficked search engines, including Google, Yahoo, Bing, Ask.com and AOL, were told that adverts placed above natural results were sometimes not clearly marked, and Ms Engle mentioned a survey showing that nearly half of searchers didn't realise these adverts were different from natural results.
Shading is often used to differentiate top adverts from other results but the letter said that the shade used was often too faint.
Vertical search options, which allow searcher to narrow their search to a specific category such as news or consumer goods, were also mentioned as the FTC said results are sometimes "based at least in part on payments from a third party."
Ms Engle wrote that any method could be use to distinguish paid adverts from search results, "so long as it is noticeable and understandable to consumers".
The FTC said this principle should be relevant even as search engines evolve to offer customers new ways of finding information.
The FTC letter did not single out any company for offences. In January, the US authority completed an investigation into whether Google was unfairly favouring its own products in search results, and found there was no need to take action.