The Competition And Markets Authority (CMA) is investigating the proposed merger between AEG Facilities and SMG.
The venue giants announced their intention to join forces to form ASM Global, a new standalone global facility management and venue services behemoth, which would operate more than 310 venues across five continents.
The CMA has launched its phase 1 investigation following the European Commission’s decision to partially refer the case to the UK, and will consider whether the planned deal would result in a "substantial lessening of competition".
AEG Facilities is the venue management subsidiary of AEG, while Onex-owned SMG is the parent company of Manchester-based SMG Europe.
The watchdog is welcoming representations from interested parties until April 29, with a deadline for the phase 1 decision scheduled for June 24.
Subject to approval, the deal will see AEG retain ownership of its real estate holdings, including its owned venues in Los Angeles (Staples Center), London (The O2), Hamburg (Barclaycard Arena) and Berlin (Mercedes-Benz Arena).
Onex is contributing its entire equity investment in SMG into the merger, which includes UK venues Manchester Arena, Leeds' First Direct Arena, Newcastle Utilita Arena and SSE Arena, Belfast.
"Opportunities like this don't come along that often and to be able to be involved with just a cutting-edge, dynamic new company to serve our industry is pretty special," AEG Facilities president Bob Newman told Music Week earlier this year.
The new company will be called Wildlife Holdings, according to the CMA document.