Google announced the change Wednesday in response to a June ruling by the European Commission that Google had abused its dominance in search to illegally advantage its comparison-shopping service. The case is one of three pending against Google in Europe, as regulators around the world more closely scrutinize the search giant's business practices.

In a letter to European regulators, however, the European Consumer Organization wrote that Google's plan to auction space at the top of search results could increase costs for consumers and won't provide "results based on merit or consumer relevance."

Maurice Stucke, a law professor and cofounder of Konkurrenz Group, which advises companies on competition issues, says "behavioral" changes like Google's planned auctions typically are not effective at curbing monopolistic behavior. "Merely telling them 'You can't do this in the future,' " may not be sufficient, Stucke says. "Now they have such an inherent advantage that even if they don't engage in this behavior, they still win."