Google's proposed $3.1bn (£1.5bn) acquisition of DoubleClick has been criticised on competition and privacy grounds by European consumer rights group Beuc.
According to reports, Beuc's director Jim Murray wrote to European Commissioner Neelie Kroes last week to warn the deal "may have a negative impact on the selection of online content available to consumers and on privacy" and mean that consumers "would have no real ability to choose services other than those served by Google".
The US Federal Trade Commission is already set to investigate the deal after rivals including Microsoft and AT&T complained it would give Google too much power in the online advertising sector.
Beuc represents 40 consumer rights groups from 14 countries within the EU and is 45% funded by the European Commission.
Its letter also indicated concerns about the impact of the deal on user privacy.
It stated: "The unprecedented and unmatched databases of user profiles will constitute significant and possibly insurmountable barriers to entry but they appear to be in clear violation of users' privacy rights."
Google acquired DoubleClick, which offers display advertising management services to advertising and media agencies and online publishers, in April this year. Google beat bids from Microsoft and Yahoo! with an all-cash offer.
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