Sky might not be the limit for Comcast Corp., which is mulling another bid for 21st Century Fox assets, a move that could upend the Walt Disney Co.'s efforts to buy much of Fox from Rupert Murdoch.

Late Monday, Reuters reported that Comcast was lining up financing to make an all-cash bid for Fox's vaunted TV and movie studio, FX, National Geographic channels, nearly two dozen regional sports networks and control of the online streaming service Hulu. A Comcast bid, should it materialise, would eclipse Disney's $US52.4-billion ($70 billion) proposal.

Shares in Rupert Murdoch's 21st Century Fox rose overnight on the speculation. Credit:Bloomberg

A new Comcast bid would be a direct challenge to Disney Chief Executive Bob Iger, whose effort to buy much of Fox was seen as a bold move to cement his legacy and secure Disney's future. The Burbank entertainment giant now might have to raise its bid considerably for Fox. Murdoch, too, would be in a tight spot because he would have to justify for shareholders accepting a lower amount when Comcast was offering more money.

Investors were encouraged that Fox might still be in play, and Fox's stock jumped nearly 2 per cent in Tuesday morning trading. Fox's shares are up nearly 50 per cent since early October. Comcast stock was down more than 3 per cent, and Disney shares were down less than 1 per cent.