The European Commission has taken the unusual step of sending questionnaires to Google customers before the company officially seeks permission to take over a rival, two business sources familiar with the situation said on Thursday.

Google is expected to file with the European Union's top antitrust regulator for the $3.1 billion purchase of U.S. Web advertising supplier DoubleClick by mid-September, the sources said.

We believe they have taken this step because the Commission believes this will be an unusually complex and contentious merger, one of the sources said.

Once Google files -- as it has in the United States before the Federal Trade Commission -- the European Commission will evaluate the deal to determine whether it is anti-competitive.

The European Commission often sends questionnaires to competitors and customers once a deal is formally filed, but it is far more unusual to provide one early.

A Commission spokesman declined to comment.

In Washington, the Federal Trade Commission has already requested additional information about the deal.

Rivals have asked authorities to look closely at the proposed buy, raising concerns that Google could gain too much control over online advertising.

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