The U.K. Competition Commission said that combining Ticketmaster and Live Nation “could severely inhibit the entry of a major new competitor, CTS Eventim, into the U.K. ticketing market.” The British regulators have until Nov. 24 to determine what actions to take.

Antitrust experts said the ruling makes it more likely that U.K. and U.S. authorities will coordinate a response. That could mean Ticketmaster and Live Nation will be forced to sell off assets, or forced to sell some concert tickets through a competitor.

Live Nation shares fell nearly 6 percent Thursday, while Ticketmaster shares fell more than 4 percent.

“I think the writing is on the wall that the deal is not going to happen without some fix,” said Mark Ostrau, an antitrust lawyer with Mountain View-based Fenwick & West LLP.

Several antitrust lawyers also noted that the U.S. Department of Justice has taken unusually long to give a green light to the deal that was announced in February.

“The fact that the U.K. regulators acted puts the U.S. more on the spot,” said Kevin Arquit, an antitrust lawyer and former director of the Federal Trade Commission’s Bureau of Competition. But he added there was “no obligation” for U.S. authorities to follow the British lead.

Before the merger plan was announced, Bremen, Germany-based CTS Eventim AG had agreed to provide ticketing services for Live Nation’s music events in Britain. CTS also has enabled Live Nation to run its own ticketing platform, which currently competes with Ticketmaster, in the U.S. since January.

The commission said if the merger were to proceed, Live Nation would have an incentive to inhibit CTS’s business in the U.K. by minimizing the number of tickets it gave CTS to sell. That could lead to higher ticket prices, the commission said.

The commission said it would consider possible remedies and “continue to consult with the U.S. authorities” before issuing its final report by Nov. 24. It set an Oct. 29 deadline for interested parties to comment.

In a joint response, Ticketmaster and Live Nation said their merger was necessary to help “strengthen a flagging music industry.” They said they continued to cooperate with U.K. and U.S. authorities on the ongoing review.

“We believe this merger will build a more efficient and effective company moving forward … and remain optimistic that it will ultimately be approved,” they said.

Analyst David Joyce of Miller Tabak & Co. said he believes the deal will still go through after conditions are imposed on both sides of the Atlantic.

“The companies went into this knowing they’d have to compromise somewhere,” Joyce said. But uncertainty about the conditions will hurt the companies’ shares for now, he added.

Ticketmaster, based in West Hollywood, is the largest seller of tickets to live concerts and other entertainment events in the United States. It also owns an artist management company, Front Line Management, and ticket-reselling company TicketsNow.

Live Nation, based in Los Angeles, is the world’s largest concert promoter. It also owns more than 140 entertainment venues.

Joyce said Ticketmaster could meet regulators’ concerns by selling TicketsNow. Or the companies could be forced to allot blocks of tickets to be sold through third parties.

The National Association of Ticket Brokers, a Washington, D.C.-based lobbying group, hailed the British ruling. Ken Solky, the association’s president, said that Ticketmaster and Live Nation could control every part of a live concert, from artist management and promotion to venue management, merchandise sales and ticket sales and resales.

“Any time somebody’s trying to eliminate any and all choices for the buying public, you’re going to have the same concerns whether it’s here or abroad,” Solky said.

Aside from the U.S. Justice Department investigation, lawmakers have raised concerns about the deal.

In July, 50 House members wrote to Assistant Attorney General Christine Varney, urging Justice officials to examine the merger “with great skepticism” because the combined company “would enjoy a virtual stranglehold over the live entertainment industry.”

U.S. Rep. Bill Pascrell Jr., a New Jersey Democrat who introduced a bill in June to clamp down on ticket brokers, said he hoped Thursday’s decision would prompt U.S. regulators to act on the Ticketmaster-Live Nation deal as well.