UPDATE 1-Comcast crosses 30 percent Sky threshold after buying shares
Crossing the 30 percent threshold means that the U.S. cable company must offer to buy out other investors at the formal offer price of 17.28 pounds per share, valuing Sky at around 30.6 billion pounds ($40.2 billion).
Shares in Comcast, owner of Universal Pictures and NBC network, fell on Monday on concerns about how much the company had paid to clinch the deal.
Fox holds a 39 percent stake in Sky, stemming from Murdoch's role in its creation nearly three decades ago, which it is selling to Walt Disney as part of a separate deal.
Neither Fox nor Disney have said what they intend to do with the stake.
Comcast's winning bid quickly received the backing of Sky independent directors on Saturday. They said Comcast's offer was "materially superior" to Fox's, and they urged shareholders to accept it immediately.
Sky's shares were trading at 17.27 pounds in early deals on Tuesday.
Comcast needs 50 percent of the stock plus one share to complete its takeover. It is hoping to wrap up the deal by the end of next month.