Sky's chairman is Murdoch's son James, who is the chief executive of 21st Century Fox, so Comcast will have to gain the support of the independent shareholders for its better offer if it does not make a hostile bid.
Britain's top share index was up 0.13 percent at 0943 GMT, with Sky soaring 20 percent as investors priced in a possible bidding war for the company following Comcast's surprise move. "We look at Sky as a media company", rather than a satellite TV distributor.
The NBC logo and Comcast are displayed on 30 Rockefeller Plaza, formerly known as the GE building, in midtown Manhattan in New York July 23, 2015. He said Sky's chief executive Jeremy Darroch had shown him an early preview of the Sky Q TV platform box, and that several people had worked as executives for both Comcast and Sky. The 86-year-old mogul therefore has two options: revise the terms of the Disney sale to exclude Sky, or enter into a bidding war with Roberts.
Comcast, which owns cable channels MSNBC and CNBC, believes its approach would escape concerns over media plurality because of its "minimal presence" in United Kingdom media.
But regulators are holding up the bid and Sky shareholders could be tempted by a higher offer.
Driven by Murdoch's expansion ambitions, 21st Century Fox has long tried to buy the 61% of Sky it does not already own. The superior cash proposal values each Sky share at £12.50 in cash-a significant premium to the 21CF price now recommended.
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Fox would hand full control of Sky to Disney if its takeover is successful. The regulator had suggested spinning off Sky News and insulating it from Murdoch's influence might be one potential remedy.
Disney saw the Fox acquisition as a way to quickly bulk up to take on Netflix, which has been growing rapidly with strong sales in overseas markets.
Comcast has also promised to maintain Sky News, one of the underlying issues that has stalled the 21st Century Fox bid.
"While Comcast does own a substantial worldwide operation in the United Kingdom".
"Indeed the fact that Sky shares were already trading at 30p above Fox's offer price tells us the market was expecting an improved offer from somewhere".
Anne Lambert, chair of the independent investigation group, said: "Media plurality goes to the heart of our democratic process".
Given the strategic importance of Sky to Fox and Disney, a counteroffer well above Comcast's is now very likely, said Jerry Dellis, an analyst at Jefferies in London.
"While Comcast does own a substantial global operation in the United Kingdom, with more than 1,300 employees, the Company has only a minimal presence in the United Kingdom media market", Comcast said in its bid document.