The announcement came four days after the U.S. cable giant won majority ownership of Sky in an auction against Fox.
Comcast and Fox had been embroiled in a bidding war for Sky, which is viewed as a coveted international asset in the pay-TV industry. Fox sold most of its other assets in December to Disney for $52 billion. Comcast has anticipated that Fox, working in concert with Disney, would eventually tender its stake, sources familiar with the matter told CNBC this week.
With Comcast having offered $40 billion for a majority stake in Sky, Disney consented to 21st Century Fox tendering its 39 percent stake in Sky to the cable company, which owns CNBC parent NBCUniversal. At current exchange rates, the 17.28 pound offer translates into $22.75 per share, valuing the deal for the remainder at over $15 billion.
"The transaction, coupled with the divestiture of the Fox Sports Regional Networks, will significantly reduce the amount of debt Disney will incur in acquiring 21st Century Fox, and enable Disney to maintain its strong balance sheet as it continues to invest in content creation for its direct-to-consumer platforms," Disney said in a statement.
Disney could have gained control over Sky had Fox prevailed at Saturday's auction.
The proceeds from 21st Century's Sky stake will go to Disney, helping to alleviate the cost of its $71.3 billion acquisition of a variety of Fox assets. These include Fox's film and TV studio, several cable stations, a stake in Hulu, Star India and other assets.
Meanwhile, Comcast plans to allow Sky to operate independently for the time being, according to a person familiar with the matter, who asked not to be named because discussions are private.
"With 21CF announcing its intention to sell its shares to Comcast we close one chapter while simultaneously opening another," said Jeremy Darroch, Sky Group's CEO in a statement. "Our aim is to make the next 30 years as exciting for customers, colleagues and all our stakeholders."
The British broadcaster has recommended to its shareholders that they accept Comcast's offer immediately. However, the deal won't be confirmed until Sky shareholders formally accept it.
--CNBC's Alex Sherman, Anjali Robins and Graham Smith contributed to this article.