Still, the ministry reserves the right to rule on prominent deals. And in this case, the acquisition is likely to cut down on incentives for drivers. That could mean higher costs to users, who have become accustomed to rides across China’s traffic-soaked cities that — thanks to subsidies — can cost much less than a regular taxi fare.

“Since there’s been considerable popular concern — because this is an industry that affects ordinary people — it wouldn’t be surprising if the Ministry of Commerce will feel an obligation to scrutinize the transaction very carefully and possibly impose conditions,” said Lester Ross, a partner at the Beijing office of the law firm Wilmer Hale.

The investigation comes as China looks to bolster its antitrust enforcement. The country’s antitrust law is only eight years old, but local experts say it is an essential part of nurturing a stronger consumer culture and greater competition in business. Overseas companies have sometimes complained that antitrust authorities unfairly focus on them, an accusation that China denies.

In his statement, Mr. Shen of the Commerce Ministry said that Didi did not initially notify the agency that it had struck the deal. He said the ministry held two subsequent interviews with Didi, asking the company for further explanations for the reasons behind the deal and to broadly describe industry trends.

The stakes are high for Didi and Uber. The ministry could put conditions on the deal or even block it outright.

Although a rejection is unlikely given the buyer is a Chinese company, there is little precedent for the merger. And the ministry has held up transactions, sometimes for years, to extract the terms it desires. In this case, Chinese regulators could push for pricing controls or find other ways to slow the pace of expansion of the new Didi-Uber company. Drivers who bought cars expecting a steady payout of subsidies could also be affected.