The Competition And Markets Authority (CMA) has opened an inquiry into Viagogo's $4 billion acquisition of StubHub.
The CMA is investigating whether the proposed deal would be "expected to result in a substantial lessening of competition within any market or markets in the United Kingdom".
The watchdog is inviting comments on the transaction from any interested party up until the deadline of January 10, 2020.
A Viagogo spokesperson said: “It’s customary for regulators to review acquisitions of this size. We will work collaboratively with the CMA to address any of the enquiries.”
FanFair Alliance, which campaigns against industrial-scale online ticket touting, has 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.
Ticketmaster UK closed its resale sites Seatwave and Get Me In! last year in favour of launching a new fan-to-fan ticket exchange in December, which left just StubHub and Viagogo of the so-called Big 4 secondary ticketing sites.
The CMA had previously prepared to take the Viagogo to court over accusations of failing to comply with an order over the treatment of customers but later dropped its action, satisfied the secondary ticketing site had addressed its outstanding concerns.