The telecommunications giant said Wednesday it is in "advanced stages of preparing" a "superior" all-cash offer for parts of Fox. Disney agreed in December to acquire those assets, which include Fox's movie studios and a stake in Sky, for $52.4 billion in stock.
However, Disney's stock offer would allow Fox to spin off the assets tax-free, while a cash offer from Comcast would result in a taxable spin, sources said.
A deal with Fox would boost Disney's efforts to dominate video streaming and compete with Netflix.
Shares of Comcast closed 1.9 percent lower, while Disney fell 1.1 percent. Fox shares gained 1.6 percent.
Disclosure: Comcast is the owner of NBCUniversal, parent company of CNBC and CNBC.com.