The FanFair Alliance has written to the Competition And Markets Authority (CMA), urging them to investigate Viagogo's $4 billion acquisition of StubHub.
Viagogo said the deal "will unite the two businesses which share the same fundamental principle of providing a secure platform for people to buy and sell tickets to live events".
But FanFair Alliance, which campaigns against industrial-scale online ticket touting, has raised competition concerns over the deal, alleging it would give Viagogo a monopoly over the "for-profit" secondary ticketing market in the UK.
"There may be different perspectives in North America, where there is a competitive market in 'for profit' secondary ticketing; but this is not the case in the UK, and we believe a merger of Viagogo and StubHub would be firmly against the interest of UK consumers and risk audiences paying needlessly inflated ticket prices," said a FanFair Alliance spokesperson.
"Furthermore, it would concentrate market power in 'for-profit' secondary ticketing in the hands of a single operator (a combined Viagogo/StubHub would control closer to 100% of the UK market, far above the CMA’s 40% benchmark) and potentially result in anti-competitive behaviour with significant and damaging implications throughout the UK’s live music sector."
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.
"A merger of the two, would potentially leave a single market-dominant platform," added the letter. "There would be no competition.
"This would, we believe, inevitably lead to even higher fees for consumers, and an even greater dominance of search and social media advertising - increasing the risk of consumers being led needlessly towards inflated tickets in the secondary market, and away from authorised primary ticket agents."
Viagogo returned to Google's paid-for search rankings this week after the web giant suspended the site back in July after finding the Swiss-headquartered company in breach of its advertising policy.