Deborah Feinstein, head of the FTC’s Bureau of Competition, is opposing the deal, according to a story in the New York Post, which cited an unidentified source. The FTC commissioners are expected to rule by October, though Staples could ask for the deadline to be extended, the Post said.

Staples’ stock fell 5.8 percent to $12.75 in the wake of the story, marking its biggest decline in seven months. Office Depot dropped 4.1 percent to $7.06.

“Things aren’t looking great for the deal, but I don’t think the fat lady has sung,” said Anthony Chukumba, an analyst at BB&T Capital Markets.

A merger of Staples and Office Depot would leave the U.S. with one major office-supply chain, raising antitrust issues. The FTC has been studying the impact on large corporate customers, people familiar with the matter told Bloomberg News last month. While consumers can pick up paper and pens anywhere, there are fewer options for companies that buy products in large quantities and depend on steady deliveries and discounted prices.

The most appropriate remedy is for Staples to divest a significant portion of its delivery business, Chukumba said.

Staples, based in Framingham, Massachusetts, first announced plans to buy Office Depot in February for about $6 billion. Representatives for the two companies didn’t respond to requests for comment on Tuesday.