Tuesday, the BEUC, a group of consumer organizations in Europe, sent a letter outlining concerns about the merger to the European Commission, currently evaluating the deal.
"The Google/DoubleClick merger would harm consumer welfare by creating a structure that almost certainly will be less respectful of user privacy," the letter states. "A combined Google/DoubleClick will be a data collection colossus that combines information about consumers that Google collects through its search engine with the tracking data that DoubleClick collects about users as they surf the web."
Google did not respond to a request for comment.
Last month, the EU, which has until April 2nd to render a decision, said that the deal raises concerns warranting further investigation.
If European authorities don't approve the deal, the companies won't go through with the merger even if the FTC clears it.
Meanwhile, consumer advocates in the U.S. continue to urge that the FTC block the merger. In an 18-page letter sent to the FTC last week, Jeff Chester, founder and executive director of the Center for Digital Democracy, reiterated concerns that a combined Google/DoubleClick entity would dominate online advertising.
"Given the scale needed to compete with a combined Google/DoubleClick, there will be insurmountable barriers to entry in the interactive ad market," he wrote. "There are powerful network effects, involving the collection, analysis and targeted use of consumer and business behavioral data sets, that will be compounded if the merger is approved, effectively eliminating whatever competition is currently viable in the marketplace."
Chester also urged that if the deal is approved, it should be with conditions that include a five-year period during which DoubleClick would have to operate as an independent entity.