The announcement comes two months after Alistair Darling, the then secretary of state for trade and industry, referred BSkyB's acquisition to the commission for a full investigation.
The commission said it would look at "plurality issues" in the provision of news and current affairs.
ITV has a 40% stake in ITN, which last year lost the contract to supply Five with news after rival Sky News won the contract.
The commission said it might extend this line of enquiry to examine the impact of the acquisition on "different aspects of culture more generally".
At the time of BSkyB's acquisition, during which it spent nearly £1bn on the shares, the Office of Fair Trading and Ofcom said the stake raised competition concerns in terms of a lessening of competition in the market.
The commission said in its statement that it would look into competition issues and will decide whether Sky's stake gives it material influence over ITV.
The investigation will also attempt to construct what would have happened in the absence of the acquisition, an issue that will interest Sir Richard Branson and Virgin Media.
The acquisition of the stake has been interpreted as a blocking move by many, because it curtailed Branson's ambitions to merge Virgin Media with ITV.
The Competition Commission's statement has been made to identify for all interested parties, such as ITV and BSkyB, the issues and specific questions the investigation will examine.
It now has 24-week period in which to complete its report, which may be extended by no more than eight weeks if it considers that there are special reasons why the report cannot be completed within that period.
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