The commission had also proposed the option of forcing Sky to sell its entire stake, but said it preferred a partial divestment as "least intrusive" and a "more proportionate" option.
Sky, which is expected to mount an appeal against the decision, said it would "give careful consideration to the announcement and confirm any further steps in due course". It has until February 25 to make any appeal to the Competition Appeal Tribunal.
ITV said it "warmly" welcomed the decision, describing it as "in the best interests of the overwhelming majority of our shareholders".
Sky won a small victory in persuading the government not to disclose the period of time within which the divestment of shares has to be completed. John Hutton, the secretary of state for business, sided with Sky on the issue, despite ITV calling for the information to be made public.
Sky now faces the problem of how to minimise the losses it will take from unwinding the greater part of its £940m investment in Sky. Having paid 135p a share in November 2006, it would lose £250m assuming it sold the required 10.5% at this morning's ITV share price of 73p.
Nevertheless, the purchase was satisfactory for the company in that it frustrated its pay-TV competitor Virgin Media's attempts to merge with the UK's largest commercial broadcaster.
Amid protests from Sir Richard Branson, Virgin Media's major shareholder, and concern from public figures and Michael Grade, the ITV executive chairman, the government and the Office of Fair Trading put Sky's acquisition of the shares under scrutiny.
Then secretary of state Alistair Darling referred the issue to the Competition Commission in May and the commission delivered its report in December.
The report found that the stake resulted in a substantial lessening of competition in the UK television market.
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