The Competition and Markets Authority (CMA) provisionally ruled in January the takeover "may be expected" to act against the public interest because it would concentrate too much influence over the United Kingdom media industry in the hands of the Murdoch Family Trust (MFT).
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.
Any deal Disney is required to make for Sky would cost the same as Fox's bid for the operator and would only be required if Fox doesn't close its deal for Sky first.
It means that even if Fox's plans to take over Sky are blocked by competition regulators, Disney would still have to offer Sky's shareholders a deal at the same price of 1075p per share.
If Comcast ends up acquiring Sky, Disney would not have to put forward a bid, the Takeover Panel said. "Further advice to Sky Shareholders will be announced in due course", it stated.
Today, UK authorities said Disney would not have to make the mandatory offer in one of two eventualities: if the CMA waves through the Murdoch takeover, or if Comcast's approach is firmed up and accepted by Sky shareholders.
Sky shares rose 0.1 percent to 13.11 pounds at 1:15 p.m.in London. The agreement was related to the disclosure of information for enabling Fox and Disney to assess and obtain antitrust approvals needed for the proposed acquisition of Fox by Disney including the indirect acquisition of Fox's interest in Sky.
The three companies - Sky, Fox and Disney - signed a confidentiality agreement in March.
Disney will have to make a full takeover bid for Sky even if the competition regulator quashes Rupert Murdoch's £11.7bn attempt to buy 100% of Britain's biggest pay-TV broadcaster, the United Kingdom takeover panel has ruled.