John Hutton has told broadcasting giant BSkyB it must sell some of its 19.9 per cent stake in ITV.
The secretary of state for business and enterprise has decided Sky must reduce its shareholding to less than 7.5 per cent, in line with recommendations by the Competition Commission.
The move could cost the company £250m, following sharp falls in ITV's share price.
Hutton found that Sky's stake in the group "resulted in a substantial lessening of competition within the UK market for all television".
Investigations by watchdogs Ofcom and the Office of Fair Trading had concluded that the stake raised "significant" competition and public interest concerns, leading to the Competition Commission inquiry.
Sky bought its stake when shares were valued at 135p each, but they have since plummeted - hitting an all-time low of 70.7p last week.
The company has four weeks to appeal the decision, but said only it would "give careful consideration to the announcement and confirm any further steps in due course".
ITV said it "warmly welcomed" the decision, which was "in the best interests of the overwhelming majority of our shareholders".


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