Now, Sky's independent directors have withdrawn their support for the Fox deal, as Comcast offers a 16 percent premium above Fox, and that sets the scene for a possible bidding war.
For its part, 21st Century Fox also issued a statement regarding the announcement on Wednesday, in which it maintained that the company is "committed to its recommended cash offer for Sky announced on 15th December 2016 and is now considering its options".
Comcast had on February 27 outlined its "possible" offer, which is now firm and trumps an offer from Rupert Murdoch's 21st Century Fox, which was made in December 2016.
Shares in Sky were trading up 3 percent at 13.46 pounds at 1026 GMT.
Comcast, the owner of NBC and Universal Pictures, said it was delighted to be formalising its offer.
Comcast has put an offer of £12.50 per share on the table as a part of its bid.More news: Warriors trailing Spurs in Game 4 of NBA playoffs
More news: Prescription drug take-back events set for Saturday across Utah
More news: This Taco Truck In Milwaukee Is Ready For Giannis Antetokounmpo
Comcast said in a statement that it would establish an independent board for Sky's news channel, and would commit to funding it for 10 years.
After Comcast's formal bid, Sky withdrew its recommendation that shareholders should accept Fox's offer. But Comcast isnt wooing just Sky shareholders. The contest pits Iger against Comcast CEO Brian Roberts, after Disney won over Comcast in an earlier battle for Fox.
Comcast said its cash offer is to buy the remaining 61 per cent of Sky it does not already own.
Murdoch's British media assets, which include newspapers The Sun and The Times, already reach a third of the country's news consumers.
Comcast said its bid came with a series of legally binding commitments on Sky ownership and United Kingdom investment.
As a result of the intrigue over Sky, Comcast and Fox/Disney could end up fierce competitors, partners, deal-makers - or all three.
Other TCI officials have also had conversations with Comcast's investor-relations team that left Comcast executives with the clear indication that TCI wants the cable giant to continue its pursuit of Fox, the people said.