Chain store sales (search) fell for the third consecutive week, as on-line shopping and gift cards continue to cut into weekly sales figures, a report said on Tuesday.

Sales in December to-date were down 0.7 percent compared with November. Sales rose by 2.1 percent on a year-over-year basis for the week ended Dec. 18, said Redbook Research (search), an independent company.

"Some sources said markdowns had become pervasive and were affecting margins. In addition, on-line shopping remained at a strong pace and gift cards were on the rise shifting sales to the period after the holiday season which could affect sales data," the report said.

The Redbook data are compiled from a sample of same-store sales at general merchandise retailers representing about 9,000 stores. Same-store sales (search) measure revenue at stores open at least one year.