Rupert Murdoch's 21st Century Fox has already offered to buy Sky, and Fox will now be considering whether it will increase its original offer of £10.75 ($15) per share to outmatch Comcast's offer of £12.50 (around $17.50) a share.
Sky shares rose 4.1 percent to 13.62 pounds by 1107 GMT - well above the offer price, implying that some shareholders anticipated a bidding war. The cable TV firm announced its plan to bid for Sky in February. Comcast said it would make a legally binding commitment to fund the rolling news channel at at least its current level for a decade. The bid represents a 16% premium on the latest offer from Murdoch's 21st Century Fox.
Fox already owns 39 per cent of Sky and made a bid to take full control of the group in December 2016.
"We have long believed Sky is an outstanding company and a great fit with Comcast", Chief Executive Brian Roberts said in a statement.
Comcast said the acquisition will enhance its free cash flow per share in the first year, excluding one-time transaction-related expenses and, within a reasonable period of time, the company expects that the return on invested capital of the acquisition will exceed the weighted average cost of capital.More news: Prince George and Princess Charlotte's adorable sibling moment
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A woman walks past the NBC and Comcast logos on 30 Rockefeller Plaza in midtown Manhattan in NY, U.S., February 27, 2018.
Yet the contest between Fox and Comcast has been complicated by a $52.4 billion offer from Disney past year to swallow most of Fox, including its existing stake in Sky. It also proposed to protect media plurality in the United Kingdom by giving a binding post-offer undertaking not to acquire any majority interest in any British newspapers for five years.
A tie-up between the two would create a business with around 52 million customers and give Comcast a platform for growth across Europe.
"At this time, the Independent Committee notes that both offers are subject to pre-conditions and neither offer is now capable of being put to shareholders", Sky said.