Google’s acquisition of DoubleClick, the largest broker of online banner advertising, will almost certainly be referred for an in-depth examination by the European Commission, advisers close to the search giant believe.

A referral to an in-depth phase two investigation by the European watchdog would be likely to hold the deal up for at least four months from October 26.

A decision to launch a full-blown inquiry would also be construed as an interim victory for Microsoft, which is lobbying fiercely against the DoubleClick deal being passed on competition and privacy grounds.

Microsoft was beaten by Google in an auction for DoubleClick this year.

It is understood that Microsoft matched Google’s $3.1 billion bid but was snubbed by DoubleClick’s private equity owner, Helman & Friedman.

Advisers close to the Google camp say that the crux of the DoubleClick case lies in whether online advertising is decided to be a discreet market, or part of a broader advertising landscape that also includes media such as print, TV and radio.

They say that that task is unlikely to be possible by the October 26 deadline set today by the commission.

“Determining whether that is the case is going to be a hugely fact and data-intensive exercise,” one lawyer said. “It is very hard to see it happening in a month.”

Google has publicly argued that the DoubleClick deal is "positive for users and advertisers and fosters competition".

Its claim that online advertising should be treated as a separate market will be given a boost by a German ruling to that effect over Microsoft’s recent acquisition of aQuantive, the online adverting group, for $6 billion.

However, Google, which accounted for nearly 40 per cent of the market in the UK last year, faces fierce opposition from rivals, chief among them Microsoft, which last week said that it would send Brad Smith, its general counsel, to give evidence at a US Senate hearing on the DoubleClick deal slated for Thursday.

The Senate hearing will examine the "risks for competition and privacy" of Google's planned acquisition of DoubleClick, the Senate Judiciary Committee website said.

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