A European consumer group has complained that Google's proposed acquisition of online ad broker DoubleClick could hurt privacy rights due to the massive amount of data the companies would hold.

In a letter, the Bureau Européen des Unions de Consommateurs (BEUC) has asked the European Commission and other authorities to investigate how the deal would affect consumers.

Google, which bought DoubleClick in April for $3.1 billion (£1.54bn), refuted the assertion that the deal will harm privacy. "Not surprisingly, we don't agree," said Rachel Whetstone, Google's director of European corporate communications, on Thursday.

The company thinks the deal will lead to lower priced ad services for both publishers and advertisers, and the delivery of more relevant ads to users, she said.

Google's deal with DoubleClick also prompted scrutiny. The US Federal Trade Commission is investigating the deal following a complaint filed by the Electronic Privacy Information Centre, the Centre for Digital Democracy and the US Public Interest Research Group.

The groups are concerned how Google will protect its data and users' privacy, including how it will handle cookies and other data that could potentially be used to identify Internet users.

The European Commission has not yet been formally notified of the deal by Google or DoubleClick, a Commission spokeswoman said.

Acquisitions can fall under the purview of national competition authorities or the European Commission, depending on where the companies do business, their turnover and other factors, the spokeswoman said.