The European Commission's four month investigation into Google's proposed $3.1bn (£1.5bn) takeover of advertiser DoubleClick starts today.

The proposed deal raises concerns about competition in the online advertising market. Rivals such as Microsoft claim that the merger would give the search giant a dominant position in the sector.

Last month the Australian regulator granted permission for the DoubleClick deal. And antitrust experts at law firm Kirkland & Ellis predict Google will win its case in the US, leaving Europe as the final hurdle.

The Commission now has 90 working days, until 2 April, to reach a decision about the future of the deal.

Google chief executive Eric Schmidt said: "We are obviously disappointed by the European Commission's decision to extend its review of our acquisition of DoubleClick.

"We will continue to work with the Commission to demonstrate how our proposed acquisition will benefit publishers, advertisers and consumers."