Smith calls for the merger to receive "close scrutiny from regulatory authorities to ensure a competitive online advertising market".
He claims that between them, Google and DoubleClick account for more than 80% of the adverts delivered on the internet. Microsoft is believed to have ducked out of bidding for DoubleClick when the price topped $2 billion.
The acquisition has attracted controversy, with observers concerned that it demonstrates a move by Google to circumnavigate media agencies and become a media company itself.
The takeover will enable Google, which already dominates the search-based advertising market, to lead the display advertising arena and further push into print, radio, mobile and TV advertising.