In September, the watchdog asked for views from interested stakeholders on whether the planned £271m deal between Mitie and Interserve would impact competition in the market.
Following that feedback, the CMA has now decided to launch an inquiry into the merger and has given notice to the two firms of its planned inquiry.
The CMA has now begun a phase 1 investigation and will announce by 24 November whether to refer the merger for a Phase 2 investigation.
When the merger deal was originally announced, the firms had hoped to complete the deal before the end of the year.