LONDON (Reuters) - Britain's takeover regulator said Walt Disney DIS.N must offer to buy all of Sky if it acquires Twenty-First Century Fox's 39 percent stake and if Rupert Murdoch's Fox is prevented from purchasing all of the European pay-TV company itself.

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Fox FOXA.O agreed an offer to buy all of Sky SKYB.L 17 months ago but is still waiting regulatory approval, while Disney has agreed to buy Fox assets, including its stake in Sky, in a separate deal subject to its own regulatory clearance.

The ruling means that if Fox’s bid to buy Sky is blocked by the government in June because of Murdoch’s media influence, Disney will have to step in to make the same offer to shareholders if and when it becomes the owner of Fox’s assets.

Disney had said it should not be required to make a bid for the whole of Sky in line with Fox’s existing offer if it bought the Fox assets, but Britain’s Takeover Panel ruled on Thursday that it must match Fox’s 10.75 pounds-a-share price.

Analysts had said Disney wanted a special dispensation to give it more flexibility on whether or when it would bid for the rest of Sky if it only bought the 39 percent 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.

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The Panel’s ruling will not stand if Fox has acquired 100 percent 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 percent in Sky.

U.S. cable company Comcast CMCSA.O said on Feb. 27 that it was considering making an offer for Sky.

Sky said it noted the Takeover Panel’s ruling, and it advised shareholders to take no further action at this time.

Twenty-First Century Fox said that under the ruling, any mandatory offer by Disney would only be required after Disney’s acquisition of Fox is completed, which Fox currently expects to occur after completion of Fox’s offer for Sky.

“21CF (Fox) remains committed to its recommended cash offer for Sky announced on 15th December 2016,” it said.

The offer was supported by revised remedies it had offered to Britain’s Competition and Markets Authority, it added.