Google's proposed merger with online advertising server DoubleClick would create a giant that would control a huge portion of online advertising and hurt the Internet, opponents of the deal told US lawmakers Thursday.

The merger, proposed in April, would create "extreme market concentration," said Scott Cleland, chairman of Netcompetition.org, an advocacy group representing large broadband providers. The merger would leave online advertisers with "no real competitive choice," he told the Senate Judiciary Committee's Subcommittee on Antitrust, Competition Policy and Consumer Rights.

Microsoft General Counsel Brad Smith argued that the marriage of Google and DoubleClick will allow Google to control 70 per cent of the search-based advertising market and 80 per cent of the online display advertising market.

Smith, whose company is still under court supervision for its own 2002 antitrust settlement with the US government, suggested regulators should reject the Google-DoubleClick merge on antitrust grounds.

"What are the economic consequences of allowing the largest company in online advertising to acquire its most significant competitor?" Smith said. "If Google and DoubleClick are allowed to merge, Google will become the overwhelmingly dominant pipeline for all forms of online advertising."