Twenty-First Century Fox has triggered a 46 day deadline to raise its bid for Sky in a battle with Comcast for control of the British pay-TV group.

Under British takeover rules, Rupert Murdoch's Fox now has until Sept. 22 to trump Comcast's 14.75 pound per share offer for Sky, which values the broadcaster 25.9 billion pounds ($33 billion), after it formalized its own 14 pounds per share bid.

Comcast gatecrashed Fox's attempt to buy the 61 percent of Sky that it does not already own earlier this year and the U.S. cable giant's latest, higher offer, which it submitted in July, has been recommended to shareholders by the broadcaster's independent directors.

Fox posted its formal offer document, without improving its price, on Tuesday, setting in motion a timetable to end the uncertainty over Sky's future by triggering a 46-day period during which both Fox and Comcast can lift their offers.

If the situation is not resolved by then, Britain's Takeover Panel can run an auction to bring the complex and long-winded transatlantic takeover battle for Sky to an end.

However, Fox may opt to walk away from the Sky deal rather than taking on Comcast, people familiar with the matter said.

Sky shares were up 1 percent at 15.355 pounds on Wednesday, signaling that investors expect the bidding war to continue.

In the document, Fox said it was switching from a scheme of arrangement to an offer. That means it can choose to lower its acceptance threshold from 75 percent of Sky's minority shareholders to a simple majority of all the broadcaster's shares, including its own 39 percent stake.

"Flipping to an offer affords Fox more flexibility should they wish to come back with a higher number," analysts at Olivetree Financial said. "A scheme is cumbersome as you would constantly need to co-ordinate your actions and intentions with the Sky board."