Comcast Corp. is planning a takeover bid for European pay-TV giant Sky PLC, putting the cable giant on a collision course with rivals 21st Century Fox and Walt Disney Co.

Early Tuesday Comcast said it was planning a £22.1 billion ($30.9 billion) offer for Sky, topping a rival bid from Fox to consolidate ownership of the U.K. broadcaster—and moving to peel away one of the most attractive overseas assets that Disney has separately agreed to buy from Fox.

...