Global, the operator of stations including Capital FM, Heart and Classic FM, snapped up GMG, the company behind the Real and Smooth brands, in a £70 million deal last year.
Competition Commission deputy chairman Simon Polito said: "The advertisers most likely to lose out from this deal are those who do not use national media agencies, and so includes smaller and medium-sized companies.
"These advertisers rely on the presence of competing commercial stations in order to negotiate a good deal."
The findings may force Global to sell stations in areas where the two firms overlap.
The commission is opening a consultation on its provisional decision, which will run until March 6.