Brad Smith, said in a statement that Google's purchase of DoubleClick " raises serious competition and privacy concerns in that it gives the Google DoubleClick".
"We think this merger deserves close scrutiny from regulatory authorities to ensure a competitive online advertising market," according to Smith.
Microsoft and Google have a history of clashes. Last year Google complained that Microsoft's Internet Explorer web browser was pushing users towards MSN but Microsoft was cleared by the US Justice Department.
"By keeping DoubleClick out of Microsoft's hands, Google keeps Microsoft out of its backyard," said David Bradshaw, a principal analyst at Ovum.
"DoubleClick's display advertising expertise complements Google's existing business, and if there is any company that can boost DoubleClick's revenue, it is Google.
"Google is currently cash-rich so spending $3bn to further both these aims seems well worth it."
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