The European Commission meted out a brutal antitrust verdict to Microsoft last week.

If Europe’s newly emboldened competition watchdog is searching for another target to sink its fangs into, it doesn’t have to look far: in the next month it is going to weigh up whether Google’s proposed $3.1 billion acquisition of DoubleClick merits a closer look.

The questions behind the proposed merger are so complex, it seems inevitable that the process with move to a so-called phase two investigation.

That will probably mean the acquisition will be held up for at least four months from October 26.

So what are the issues?

The DoubleClick case, some industry observers are suggesting, is the one in the technology sector most like that which resulted in Microsoft’s recent bashing. (The commission also has Intel, Apple and Qualcomm in its sights).

In particular, the argument goes, DoubleClick would help Google build a massive database of consumers’ online habits and advertiser spending plans.

The information mirrors, it is claimed, the code that underpins Microsoft Windows, the intellectual infrastructure on which the Microsoft empire is built – which the commission has ruled must be shared with competitors so they can build software tools that work well with Windows-powered PCs.

So Google may well be forced to share its information, too – or may even be blocked from buying DoubleClick for fear of the data it will wield as a result.

Well, perhaps.

Lawyers involved in the case point out that M&A regulation and the anti-competitive stuff are quite different – the first looks forwards to possible future abuses, the second looks back at abuses already allegedly committed.

What actually matters in the DoubleClick case is whether Google’s acquisition of Doubleclick, a technology platform for display - or banner - advertising and the biggest player in its field, will diminish the intensity of competition for advertisers.

One key question there is whether advertisers have a range of alternative suppliers to go to.

The answer here is yes, for now – Yahoo! and Microsoft, for two. But, Google’s ground is growing shakier every quarter it reports double-digit revenue growth. According to Ofcom, the UK regulator, Google accounted for nearly 40 per cent of online advertising spending in the UK last year – snapping up DoubleClick will increase that share.

Google will probably have to make the argument in response, lawyers say, that online advertising still only accounts for a small part of the advertising pie – alongside print, TV and radio.

This question – one of market definition – lies at the very crux of the matter. Furthermore, if DoubleClick and Google are judged by the commission to be direct competitors, then the merger of the two will be deemed "horizontal" and will be much harder to clear.

Here also, the argument that Google must rely on (though it doesn't make it publicly) – that online advertising is a separate market – is steadily being eroded by its own successes and ambitions. It is branching out into print, TV and radio. Meanwhile, heaps of anecdotal evidence suggests advertisers are migrating their spending from traditional formats to the web – suggesting that they are interlinked and effectively form one market.

This – the "one big pie" argument – is the line Microsoft has to push if it is to derail Google's purchase of DoubleClick.

There is, however, a problem: the world's largest software group recently benefited from a decision that said online advertising represented a separate part of the pie when its own $6 billion acquisition of aQuantive, the online advertising group, was passed in Germany.

Microsoft is said to be investing heavily in a lobbying campaign against the DoubleClick deal. Its spin doctors are going to have their hands full.

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