Google may not be able to stop Microsoft from swallowing Yahoo, but it has bolstered its arsenal with the acquisition of DoubleClick.

That $3.1 billion deal has been cleared by European regulators, the last hurdle after the U.S. Federal Trade Commission approved it in December.

Some internet and advertising companies said after the deal was first announced in April that a combination of Google—which dominates search and has become extremely profitable from placing text ads by search results—and the biggest internet advertising company would limit competition in online advertising.

But the European Commission found that the merger would not impede competition, noting that advertisers and other customers had options with Microsoft, AOL, and, yes, Yahoo.

"The commission's in-depth market investigation found that Google and DoubleClick were not exerting major competitive constraints on each other's activities and could, therefore, not be considered as competitors at the moment," the regulator said.

"The market investigation also found that the merged entity would not have the incentive to close off access for competitors in the ad-serving market, mainly because such strategies would be unlikely to be profitable."

DoubleClick places and tracks online ads, acting as a broker between portals like Google and advertisers. The acquisition of DoubleClick is expected to help Google expand beyond paid search ads and do more with banner ads and videos.