* Bidding to conclude by Saturday evening
* Fox and Comcast battling for leading European broadcaster
* Auction is rarely used process in UK takeovers (Adds analyst comment and further background)
By Ben Martin and Noor Zainab Hussain
LONDON, Sept 20 (Reuters) - A protracted $34 billion bidding war for European broadcaster Sky between U.S. rivals Comcast and Twenty-First Century Fox is likely to be settled by a quick-fire auction on Saturday.
Britain's Takeover Panel, which regulates merger and acquisition activity, said on Thursday it was preparing to take the rare step of intervening in the battle for London-listed Sky by running an auction for the pay-television business that lasts for a maximum of three rounds.
The process will start if Comcast or Rupert Murdoch's Fox, which has the backing of partner Walt Disney, are still competing for Sky by 1700 London time (1600 GMT) on Friday, the Panel said.
The auction will take place the following day in private and is slated to finish during the evening, when the regulator will announce the level of the bids submitted by the suitors.
It would mark a dramatic resolution of Sky's fate, which has been up in the air ever since Fox made its first bid for the 61 percent of Sky that it does not already own in December 2016.
Fox's takeover was held up by prolonged regulatory scrutiny of the proposed deal and was then gatecrashed by cable giant Comcast earlier this year.
Comcast currently leads Fox with a 14.75 pound a share offer for control of the pay-TV group that values it at 25.9 billion pounds ($34.2 billion) and has been recommended to investors by Sky's independent directors.
That trumped the 14 pound a share offer made by Fox earlier in July and is 37 percent above Fox's original 10.75 pound a share bid in 2016.
Sky shares edged up less than 0.1 percent to 15.81 pounds on Thursday.
"Shareholders will be delighted at how this has all played out, having seen the share price double since just before the original offer was made by Twenty First Century Fox almost two years ago," said Laith Khalaf, a senior analyst at Hargreaves Lansdown.
British hedge fund manager Crispin Odey, whose firm is a shareholder in Sky, has previously said the company should be valued at a minimum of 18 pounds a share.
"We still have the ability at the end of it to say no, sorry, you still haven't bid enough," he told Reuters on Thursday.
Fox is vying for the broadcaster with the financial support of Disney, which in June agreed a separate deal to buy TV and film assets from Fox, including its existing 39 percent Sky shareholding, for about $71 billion.
The fight for the company is part of a bigger battle being waged in the entertainment industry as the growth of Netflix and Amazon force the world's traditional media giants to spend tens of billions of dollars to keep pace.
Sky was formed in 1990 when Murdoch merged his fledgling British satellite TV service with a rival, and is a broadcaster of sports, films and TV shows.
An auction for Sky, which broadcasts to 23 million households across Europe, would be the biggest ever deal in the UK to be decided by a panel-run auction.
There have only been three British takeover situations since 2007 in which the regulator has run auctions, including the 6.2 billion-pound sale of Anglo-Dutch steelmaker Corus to India's Tata Steel, according to analysis by Reuters.
The last time the Panel invoked the procedure was in 2012 when it almost intervened in the battle for Cove Energy until Royal Dutch Shell abandoned its takeover attempt and let Thailand’s PTT Exploration & Production clinch a $1.9 billion deal for the gas explorer just before an auction began.
In the first round of the proposed Sky auction, the suitor with the lowest offer at the start of the process, which as things stand is Fox, may make a higher bid.
In the second round only the other suitor, currently Comcast, would be allowed to increase their offer. They would not be bidding blind and would know the outcome of the first round, a source with direct knowledge of the situation said.
If the second round draws a bid, the Panel will then run a third stage when both suitors will have the opportunity to sweeten their offers further in what would essentially be sealed bids. In this round, Fox and Comcast can make their offers contingent on the other making a new bid.
It is possible the fight could end with both Fox and Comcast offering the same price for Sky, the regulator said.
The Panel has not set any rules governing how Sky's independent directors should respond once the the auction is over. ($1 = 0.7569 pounds) (Noor Zainab Hussain contributed reporting from Bengaluru, additional reporting by Maiya Keidan Editing by Jason Neely/Keith Weir)