Google's manipulation of search results to promote its services over that of rivals has "resulted - and will result - in real harm to consumers and to innovation in the online search and advertising markets," claims a previously unpublished report by American regulators.
The findings, contained as part of a 2012 report by America's Federal Trade Commission (FTC), allege that Google has caused "harm to many vertical competitors".
Those who have complained include Yelp, TripAdvisor, Expedia, Microsoft and its search engine Bing. The report, which has ended up in the hands of the Wall Street Journal, also claims Google illegally took content to improve its own services.
One instance alleges Google copied Amazon's product rankings to rank its own results for product searches with reviews and ratings also gleaned from the online retailer. Google was also accused to have threatened smaller sites with scrapping their content or lifting it without permission.
The allegations have added fuel to the fire of those calling for Brussels to challenge Google's monopoly in Europe.
Speaking to the Guardian, David Wood, legal counsel to ICOMP, which represents Microsoft, electronics site Foundem and other Google complainants, said: "These revelations demonstrate that this is not about national interests but about competition problems.
"It is a fascinating insight into Google's practices. It's made public things they didn't want made public and highlighted discrepancies between what they said in public and what they actually did in the US."
Google general counsel Kent Walker commented: "Speculation about potential consumer harm turned out to be entirely wrong. Since the investigation closed two years ago, the ways people access information online have only increased, giving consumers more choice than ever before."
The FTC voted unanimously in 2013 to end the investigation and has said that the unredacted copy of the report had been accidentally released and that it was taking steps to ensure it did not happen again.