Rupert Murdoch's Fox agreed an offer to buy all of Sky 17 months ago, but is still waiting approval.
Disney and Fox are said to have accepted these conditions.
Disney will be required to make a mandatory offer to Sky shareholders within 28 days of its purchase of 21st Century Fox's assets, United Kingdom authorities said today.
The U.K. Takeover Panel on Thursday ruled that Walt Disney must make a mandatory offer to buy full 100 percent control of Sky if and when it completes its planned acquisition of large parts of 21st Century Fox, including Fox's stake in Sky.
The decision may disappoint some Sky shareholders expecting Disney to be forced into a higher mandatory offer.
The takeovers panel said that the basis of the ruling rests on the belief that "securing control of Sky might reasonably be considered to be a significant goal of Disney's acquiring control of Fox".
However, Disney argued that if Fox's attempt to buy the remainder of Sky were to be blocked, it should not be forced under takeover rules to try to do so itself. Comcast's approach would see Sky shareholder's receive £12.50 a share, a significant premium to Murdoch's offer.
Disney's bid must match that made by Sky at £11.7bn, which values each ordinary share at £10.75, according to the UK's Takeover Panel - the independent body that regulates mergers and takeovers.
Fox has also offered to buy the 61% stake in Sky it doesn't already own for a total of GBP11.7 billion, or GBP10.75 a share.
The decision from the Takeover Panel, which applies City rules on takeovers and mergers under a code created to ensure shareholders are treated fairly, disagreed with Disney's argument that it should not have to do so. The deal for that is being reviewed by United Kingdom regulators, while Comcast has also said it was ready to make a competing offer for Sky. "And they've got quite a good prize".