Google said Monday it would sell ads across all Clear Channel radio stations in the United States. This comes amid news that Google's competitors, including Microsoft, may challenge the DoubleClick deal over antitrust concerns.

Since the inception of its radio advertising program brought about by the acquisition of dMarc Broadcasting, Google has struggled to convince station owners to allow the company to sell their advertising time. The ClearChannel deal is certainly a step in the right direction.

All 675 Clear Channel stations across the country would make available time to advertisers, amounting to five percent of the total advertising space. Slots would be available at all times of the day.

Clear Channel has even made some important concessions that are good news for Google's initiative. Rather than only gaining access to leftover content, advertisers would also be able to bid for premium inventory, says the company.

Undoubtedly, Google execs are breathing a sigh of relief over the latest agreement. Up until now, radio advertisers only had access to 900 stations in 200 markets, but mostly low-budget and low-power operations outside of the major metropolitan areas.

However, Google's competitors are becoming increasingly wary over Google's power in advertising. The Wall Street Journalreported Monday that several companies including Microsoft, AT&T and Time Warner are all lobbying antitrust regulators to look closely at its $3.1 billion acquisition of DoubleClick.

Google beat out Microsoft, who early on was said to be in the running for the Web advertising provider. However, as the price tag passed $2 billion, the company was said to have lost interest, as did other early players, Yahoo and Time Warner.

Opponents of the deal argue that it would consolidate too much of the online advertising industry under a single company. Microsoft told the WSJ that as much as 80 percent of ads served over the Internet would be through Google after the close of the deal.

"This proposed acquisition raises serious competition and privacy concerns in that it gives the Google DoubleClick combination unprecedented control in the delivery of online advertising, and access to a huge amount of consumer information by tracking what customers do online," Microsoft general counsel Brad Smith said in a statement. "We think this merger deserves close scrutiny from regulatory authorities to ensure a competitive online advertising market."

To its defense, Google said it was confident that the deal would pass regulatory review, although it declined to comment on its competitors' complaints.