Google’s $3.1bn acquisition of online ad firm DoubleClick is set to be investigated by the US Federal Trade Commission.

The purchase of DoubleClick last month, is intended to give enhance Google’s online ad capabilities, but it has been criticised amid concerns over both privacy and competition.

Consumer groups are concerned that the vast amount of information Google will hold about the world’s internet users, and have complained about the lack of adequate measures taken to protect users’ privacy.

The Electronic Privacy Information Center (EPIC) recently filed a complaint in the US about this issue.

Others are concerned that the deal will further extend Google’s dominance of the online ad market to a point where competition will be threatened.

Rivals, including Microsoft and WPP, have expressed such concerns, with Martin Sorrell estimating that the Google / DoubleClick deal will give the search engine 83% of the contextual and targeted online ad market.

Analysts had predicted that an antitrust investigation would follow the Google / DoubleClick deal, and the FTC has confirmed that a preliminary investigation is under way.

A decision will be made in the next few weeks whether or not the investigation will be stepped up. It is believed that the case will not stick, as Google and DoubleClick clients are free to switch to competitors like aQuantive, recently acquired by Microsoft, or 24/7 Real Media, which was snapped up by WPP last week.

Google has expressed its confidence that its purchase of DoubleClick will stand up to scrutiny.