U.S. antitrust regulators are preparing to block Staples Inc.'s acquisition of smaller rival Office Depot Inc., the New York Post reported.

The Federal Trade Commission (FTC) may again delay its decision on the deal, seeking more concessions from Staples, the Post reported, citing two people familiar with the matter.

The FTC is "ramping up for litigation," one person told the Post. Two of the four FTC commissioners looking into the deal are believed to oppose it, the newspaper reported.

Staples' first attempt to buy Office Depot was blocked by U.S. regulators in 1997.

The deal is also being investigated by EU regulators, who have raised concerns about possible price hikes.

Staples said in October that it had agreed with the FTC to extend the review period for the acquisition. The FTC is expected to decide on the deal by Dec. 8.

Staples and Office Depot are in talks with wholesaler Essendant Inc. to transfer about $600 million of corporate contracts to satisfy U.S. antitrust concerns, the Wall Street Journal reported this month.

Office Depot and a spokeswoman for the FTC declined to comment. Staples representatives were not available for comment.

Staples' shares recently fell 1.8 percent to $12.09 in afternoon trading, while Office Depot was down 1.8 percent at $6.63.