The European Commission is investigating the possible negative effects of Google's proposal to buy online ad firm DoubleClick.

An initial investigation has already indicated that the deal "would raise competition concerns in the markets for intermediation and ad serving in online advertising," said the Commission.

When they notified the Commission about the deal in September the companies claimed that their activities didn't overlap at all. Google described itself as a provider of online ad space, and DoubleClick was described as a "supplier of technology for the delivery, management and reporting of display ads to advertisers, ad agencies and web publishers."

However, the Commission still suspects that the tie-up could stifle competition. In its in-depth probe the regulator will investigate whether without this transaction, DoubleClick would have grown into an effective competitor to Google in the market for online ad intermediation.

It will also investigate whether the merger "could lead to anti-competitive restrictions for competitors operating in these markets and thus harm consumers," the Commission said.

The Commission has until 2 April next year to reach a final decision, one year after the deal was announced.