Representing advertisers accounting for about 90 percent of global market spending, the WFA warned that “Internet advertisers have benefited from innovation generated, in part, by intense competition,” Stephan Loerke, the WFA's managing director, told MediaGuardian.

Just days ago, EU regulators extended the deadline for the inquiry into the DoubleClick takeover to Nov. 13. The original deadline was set for Friday.

Google has said it would make changes to the deal after industry concerns came flooding in, including not changing some of DoubleClick's business practices.

“Global advertisers are keen to see this competitive marketplace maintained,” Loerke said. “It is hard to tell within such a rapidly changing marketplace what impact these proposed developments might have.”

Google rival Microsoft has also been quick to speak out against the proposed DoubleClick deal. Microsoft Senior Vice President and General Council Brad Smith said in May that if the deal goes through, Google will control about 80 percent of the Internet ad market.