By Liana B. Baker

(Reuters) - Mixed martial arts entertainment franchise Ultimate Fighting Championship (UFC) has agreed to be bought by Talent agency WME-IMG in a transaction a source valued at $4 billion, reflecting the growing popularity of a once-fringe spectator sport.

The acquisition, which has one of the largest ever price tags for a sports brand, is also backed by private, equity firms Silver Lake Partners LP [SILAK.UL] and KKR & Co LP <KKR.N>, as well as investment firms owned by billionaire Michael Dell and his family.

"We’re now committed to pursuing new opportunities for UFC and its talented athletes to ensure the sport’s continued growth and success on a global scale," WME IMG Co-CEOs Ariel Emanuel and Patrick Whitesell said in a statement on Tuesday.

UFC, founded in 1993, popularized cage fighting and now holds more than 40 fights around the world every year. These events, which feature mixed martial arts and have surpassed pro wresting and boxing in popularity, are broadcast in more than 156 countries, reaching 1.1 billion households. Mixed martial arts has loose roots in a Brazilian form of fighting.

While UFC initially drew a niche following, it has now created global stars such as Conor McGregor, Anderson Silva, Georges St-Pierre and Ronda Rousey, a female fighter, who have mainstream followings and marketing deals.

The deal comes after the franchise last weekend hosted one of the biggest events in its history, UFC 200, a series of title fights, in Las Vegas.

The deal represents the biggest acquisition to date by WME-IMG, a talent agency formed in a 2013 merger of William Morris Endeavor Entertainment and IMG Worldwide. Last year, it acquired the Miss Universe organization from Donald Trump and rodeo-events company Professional Bull Riders.

UFC would be the latest live-events media brand with strong ties to Las Vegas to end up owned by private equity firms. Cirque du Soleil, the global circus and entertainment company, was sold to Texas-based TPG Capital LP last year in a deal valued at about $1.5 billion.

WME-IMG did not provide the terms of the deal in a statement, but a source confirmed the deal values the mixed martial arts franchise at around $4 billion. Previously, the largest sports deal was Liberty Media Corp's purchase of the Atlanta Braves in 2007 for $2.4 billion, according to Thomson Reuters data.

UFC's parent, Zuffa LLC [ZUFFA.UL], which gets its name from the Italian word for "fight," was previously controlled by casino moguls Frank and Lorenzo Fertitta. The Fertitta brothers, who bought UFC for $2 million in 2001, will hold a passive minority stake in the company after the deal closes.

UFC's chief executive and chairman, Lorenzo Fertitta, will no longer be involved in the company's day-to-day operations. Dana White, UFC's outspoken president, will remain in his post.

ESPN first reported in May that the franchise was for sale and had drawn interest from a number of parties including WME-IMG and Chinese firms China Media Capital and Dalian Wanda Group.

Barclays <BARC.L>, Credit Suisse <CSGN.S> and Deutsche Bank <DBKGn.DE>, Goldman Sachs <GS.N> and KKR Capital Markets advised the buyers and provided financing for the deal, while The Raine Group and JP Morgan Chase & Co <JPM.N> were financial advisers to the sellers of UFC.

(Additional reporting by Narottam Medhora and Anya George Tharakan in Bengaluru; Editing by Tom Brown and Steve Orlofsky)