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.
The offer would also not be required if Comcast, which announced an offer for Sky earlier this year, or any other party has already acquired more than 50% of Sky's shares by this point.
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 UK regulator said Disney would have to make a fixed GBP10.75 per share cash offer for Sky, the same price offered by Fox.
The new regulatory requirement adds yet another wrinkle to an already complicated acquisition process that involves Disney, Fox, Sky and Comcast.
"21CF remains committed to its recommended cash offer for Sky announced on 15th December 2016, which is supported by revised remedies recently offered to the Competition and Markets Authority (CMA) with whom 21CF has been co-operating in order to bring the United Kingdom regulatory process to a swift and satisfactory conclusion", Fox said in a statement.
It is understood that Sky lobbied the Panel to force Disney to bid, allowing its independent directors, led by deputy chairman Martin Gilbert, another potential opportunity to negotiate a higher price. The country's antitrust regulator-The Competition and Markets Authority-in January provisionally ruled the Fox offer against the public interest on media plurality grounds.
The three companies - Sky, Fox and Disney - signed a confidentiality agreement in March. The Group said further advice to its shareholders will be announced in due course.
Earlier in April, Fox proposed two remedies to address plurality concerns voiced by United Kingdom mergers regulator the Competition & Markets Authority.