The proposed merger of Ticketmaster and Live Nation is to be referred to the competition commission in the United Kingdom.
The Office of Fair Trading (OFT) has expressed concern that the deal, which would unite the world's biggest ticket seller and concert promoter, would have an unfair impact on competition.
The reported $575million agreement between Ticketmaster and Live Nation was first mooted in February.
A host of musicians, including Seal and Shakira, have already endorsed the proposed merger.
But concerns in the US, and now in the UK, over whether it would harm other ticket sellers could delay the agreement.
Both Live Nation and Ticketmaster have already been asked to justify the move to the US Subcommittee on Antitrust, Competition Policy and Consumer Rights.
Live Nation has blossomed over recent years from its origins as a concert promoter to incorporate high profile partnerships with artists, including Jay-Z and Madonna.
The so-called 360 degree deals mean the company controls everything from an artist's tours to music rights.
The Competition Commission is expected to rule on the proposed deal by November 24, according to the BBC.
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