LONDON (Reuters) - Comcast Corp, the biggest U.S. cable operator, said on Tuesday it was making a cash offer to buy Sky for 22.1 billion pounds ($31 billion), challenging Rupert Murdoch's Fox to take over the European pay-TV group.
21st Century Fox had already agreed a deal to buy the 61 percent of Sky it does not already own.
($1 = 0.7157 pounds)
(Reporting by Kate Holton, editing by Guy Faulconbridge)