"Google has said it finds Microsoft's $44.6bn ($22.65bn) bid to buy rival Yahoo "troubling" and wants regulators to scrutinise the proposed deal.
In a blog, Google said the tie-up could unfairly limit the ability of consumers to freely access competitors' email and instant messaging services. It said Microsoft had previously sought "to establish proprietary monopolies".
Microsoft made an unsolicited offer for Yahoo on Friday, and Yahoo has said it is considering the proposal.
Microsoft's Kevin Johnson said that the combination of the two companies would create an entity that could better compete with Google."