Jay-Z is among the many artists signed to Live Nation
The UK Competition Commission has ruled against the merger of Ticketmaster and concert promoter Live Nation.
In a provisional ruling, it said it was concerned about the joining of the world's largest ticketing firm and the world's biggest concert promoter.
Such a deal would limit the development of competition in the market for sales of tickets to live music, it said.
The US Justice Department is also investigating the proposed merger - which was announced in February.
The two companies have said the merger would improve ticket price options, increase attendance at events and make ticket technology better.
Live Nation not only operates several large UK concert venues, but also manages a string of high-profile artists.
The company has signed the likes of Jay-Z and Madonna to deals that include touring, publishing and albums, as well as huge concert promotion deals with the likes of U2 and Shakira.
The watchdog said that its main concern was based around an agreement that Live Nation had entered into with Europe's largest ticketing agent, CTS.
Live Nation has historically used Ticketmaster as its principal ticketing agent, but this agreement expires in December.
The commission said that the deal with CTS would increase competition among ticket sellers.
And it argued that if the merger went ahead, there was a danger of competition being reduced because Live Nation would "seek to limit" its relationship with CTS - limiting how many tickets it made available.
This could force up ticket prices, lead to poorer customer service and may have "the effect of putting CTS's future prospects in the UK in considerable doubt", it said.
The watchdog will now consider ways in which its concerns about the loss of competition could be addressed, before issuing its final report.
Live Nation and Ticketmaster said they were committed to the merger going ahead and were confident it would eventually be approved.
"Where the recording industry was once the economic engine for the music business, it is live entertainment that is now the future of the music industry," the firms said, adding the deal would "help achieve needed change".