The Competition And Markets Authority (CMA) has served an enforcement notice to Viagogo and StubHub as it investigates the proposed merger between the two resale sites.
The watchdog said it had "reasonable grounds for suspecting" the businesses were already merging prior to the conclusion of its inquiry into the $4 billion deal.
The CMA's order prohibits any action that may "lead to the integration of the StubHub business with the Viagogo business"; "transfer the ownership or control of the Viagogo business or the StubHub business or any of their subsidiaries" or "otherwise impair the ability of the StubHub business or the Viagogo business to compete independently", except with its prior written consent.
The firms must also ensure "the StubHub business is carried on separately from the Viagogo business and the StubHub business’s separate sales or brand identity is maintained", while no changes must be made to key staff, among other requirements.
Executives from the two companies will be required to provide a statement confirming compliance with the order every two weeks from February 21 while the CMA concludes its investigation.
The CMA opened an investigation into the proposed $4 billion deal in December to consider whether it would be "expected to result in a substantial lessening of competition within any market or markets in the United Kingdom".
FanFair Alliance, which campaigns against industrial-scale online ticket touting, previously raised competition concerns over the deal and urged the CMA to investigate, alleging it would give Viagogo a monopoly over the "for-profit" secondary ticketing market in the UK.
Viagogo MD Cris Miller said: “The requirement to hold separate the two businesses of Viagogo and StubHub is an expected part of the merger process and we fully acknowledge the importance of the CMA’s examination into the deal.
"The acquisition has received regulatory approval in the US, but the businesses will remain separate globally, allowing the CMA to complete its inquiry and consider our evidence that this deal is a positive move for fans. We will continue to work closely with them through this process, with a view to successful completion soon.”
