In January, the Competition and Markets Authority (CMA) launched an investigation into the supermarket takeover.
CD&R, which also owns petrol station giant Motor Fuel Group (MFG), won a lengthy auction to buy the Bradford-based retailer in October.
The regulator said it now has concerns over 121 local areas where MFG and Morrisons both have forecourts and would face “limited competition” from other players following the merger.
Colin Raftery, senior director of mergers at the CMA, said: “Prices for petrol and diesel have recently hit record highs, which makes it even more important that we don’t allow a lack of competition at the pump to make the situation worse.
“We’re concerned that this deal could lead to higher prices for motorists in some parts of the country.
“But if CD&R and Morrisons are able to address these concerns, then we won’t need to move on to an in-depth investigation of the merger.”