Office Depot ( ODP) said third-quarter same-store sales likely will be weaker than expected because back-to-school shoppers are spending less than the office supplies seller anticipated.

The Delray Beach, Fla., company said in its midquarter update that North American same-store sales for the third quarter will decline in the low single digits, but Office Depot added that trends have improved in recent weeks.

The company remains comfortable with Wall Street's current third-quarter earnings forecast of 27 cents a share as cost controls and continued strength in gross margins have offset sluggish sales at stores open at least a year. Office Depot also said it was on track to meet analysts' full-year consensus earnings estimate of $1.01 a share.

Same-store sales are normally the standard by which a retailer's performance is measured. Office Depot did say that total company sales should rise in the low- to mid-single-digit range in the third quarter.

"Retail gross margins are holding, and the gains we reported last year are indeed sustainable," the company said in a press release. "We continue to find ways to take costs out of our store operating expenses; and as a result, we expect to report retail operating margins for the third quarter that are well above last year."

Shares of Office Depot were down 3% to $12.97 in early trading on the New York Stock Exchange.