Yahoo (NSDQ: YHOO) and Google (NSDQ: GOOG) have elected not to seek the European Commission’s permission for their ad sharing deal - but that doesn’t mean the proposal won’t face continental scrutiny.

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As the US Department Of Justice reportedly began a full investigation in to the plan, a spokesperson for EC competition commissioner Neelie Kroes today told paidContent:UK: “The companies in question have not yet notified their merger to the European Commission and it is up to them to do so.”

Whilst the deal, under which Google would supply ads on some Yahoo search pages, applies only in the US and Canada, some UK agencies last month voiced concern, and the EC spokesperson pointed me to guidelines that state its “rules apply to all mergers no matter where in the world the merging companies have their registered office, headquarters, activities or production facilities”. After all, the EC has fined Microsoft (NSDQ: MSFT) repeatedly.

In theory, that could leave the door open for the commission to retroactively review the tie-up. The EC defines a “merger” as companies “combining forces” and seeks to act against reductions in choice and innovation, and against the possibility of companies combining to hike prices. Yahoo said on June 12 it is “not required to receive regulatory approval”