Google's $3.1 billion purchase of DoubleClick has hit a snag, with European regulators refusing to approve the deal.

The European Commission cited antitrust concerns in ordering an additional review of the deal.

"We are obviously disappointed," Google CEO Eric Schmidt said in a statement. "We seek to avoid further delays that might put us at a disadvantage in competing fully against Microsoft, Yahoo!, AOL and others whose acquisitions in the highly competitive online advertising market have already been approved."

Since Google announced the purchase of DoubleClick in April, the company has been under fire from privacy advocates and regulators raising antitrust concerns.

In September, news surfaced that some of the opposition to the deal had come from a PR firm secretly hired by Microsoft.

The deal still needs approval in the U.S.