Google has completed its $3.1bn (£1.5bn) takeover of online advertising firm Doubleclick after the deal won support from the European Commission.
Other advertisers are worried Google could dominate the internet
Following an in-depth investigation, Brussels ruled that the merger would not be damaging to competition despite bitter protests from rivals.
It follows approval from US regulators in December.
The online advertising market is expanding and Google, Yahoo and Microsoft are battling for dominance.
Google allows firms to tailor advertising based on the words or phrases people use when searching the web and also stores information about users' internet surfing habits.
It hopes to beef up its presence in the online ad sector with its purchase of Doubleclick, which allows marketing firms, advertising agencies, and website publishers to find each other.
"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 Commission said in a statement.
Some of its rivals disagreed with the Commission's decision.
"Today's acquisition of DoubleClick will provide Google with unrivalled access to consumer data and a foothold in the display media space," said Wayne Arnold of the Institute of Practitioners in Advertising (IPA).
"As a result, advertisers and agencies will require strong reassurance that this new market power will not be exploited at the expense of consumer privacy or unfair market dominance."
Microsoft is trying to loosen Google's grip on the internet, where its innovative approach has seen it become dominant in numerous applications not just online advertising.
In an effort to challenge Google, Microsoft has tabled an offer to buy Yahoo.