The UK's takeover regulator ruled today that Walt Disney must make an offer for the whole of Sky if it succeeds in buying Twenty-First Century Fox assets.
This would include its 39% stake in the European pay-TV company.
The Takeover Panel also said Disney must match Fox's £10.75 a share offer for the shares in Sky it does not already own.
Rupert Murdoch's Fox agreed an offer to buy all of Sky 17 months ago, but is still waiting approval.
In the meantime, Disney agreed to buy Fox assets, including its stake in Sky, in a separate deal, which is subject to regulatory clearance.
Disney had said it did not believe it should be required to make a bid for the whole of Sky in line with Fox's existing offer if it bought the Fox assets.
Analysts had said Disney wanted a special dispensation to give it more flexibility in terms of whether or when it would bid for the rest of Sky if it only bought the 39% stake from Fox.
The Takeover Panel, however, said it considered that securing control of Sky might reasonably be considered to be a significant purpose of Disney's acquiring control of Fox, and it must make an offer within 28 days of buying the Fox assets.
The panel's ruling will not stand if Fox has already acquired 100% of Sky by the time Disney buys the Fox assets, or if Comcast Corp or any other third party has acquired a stake of more than 50% in Sky.
US cable company Comcast said on 27 February that it was considering making an offer for Sky.
Sky said it noted the ruling, and it advised shareholders to take no further action at this time.