In a move drenched in irony, Microsoft has complained that Google's acquisition of DoubleClick could put it in a monopolistic position in internet advertising.
Microsoft, which lost out to Google in an auction for DoubleClick, has spent years fighting charges of monopolistic activity itself, so is in a strong position to recognise a monopoly when it thinks it sees one.
Brad Smith, Microsoft general council, said: "This merger deserves close scrutiny from regulatory authorities to ensure a competitive online advertising market."
A Google spokesperson said: "The advertising market is extremely competitive and dynamic and with or without DoubleClick, Google serves only a tiny fraction of the market."
Some estimates put the combined company's share of the online advertising market at 80 per cent. The full investigation expected to be demanded by Microsoft will, even if it rules in favour of the deal, tie it up for months.