WPP chief executive Martin Sorrell has expressed concern at Google's increasing might in the advertising sector following the search giant's £1.5bn purchase of DoubleClick.
London-based WPP is the world's second-largest advertising group, but, after reporting a 0.7% drop in turnover on Friday, Sorrell told Reuters: "It raises some issues for us."
"It raises issues as to whether we are happy to let Google have our client's data and our own data which Google could use for its own purposes in contextual and targeted advertising."
Sorrell warned estimates showed Google would control 83% of the contextual ad market.
He said WPP's Google spend increased from $150m to $200m in the last year but said Google "is a short-term friend and a long-term enemy".
WPP is not the only competitor of either firm to have voiced concerns over the acquisition.
But perhaps the biggest threat to the deal could come from regulators now that online liberties groups have filed objections.
A posse led by the Electronic Frontier Foundation on Friday lodged a complaint with the US Federal Trade Commission and sought an injunction, claiming the buyout would break limits on the amount of data advertisers can collect on consumers.
"Google's proposed acquisition of DoubleClick will give one company access to more information about the internet activities of consumers than any other company in the world," the complaint says.
DoubleClick denied Google would unite the two companies' various data collections.