Brick-and-mortar retailers saw no signs of relief last week, as store traffic in the final week before Christmas posted the third straight week of double-digit declines, according to the most recent report from ShopperTrak. According to the analytics firm, traffic for the week ended Dec. 22—which included the crucial final weekend before Christmas—was down 21.2 percent year over year. The first two weeks of December saw double-digit decreases, which trailed a 4 percent decline over Black Friday weekend, it said. In-store sales fell 3.1 percent from the same week in 2012, ShopperTrak added.

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"I think the Black Friday and Thanksgiving Day sales took a lot of energy out of the consumer," said ShopperTrak founder Bill Martin.

"We still have some days left to go—there could be some ground made up. [Monday] was an important day, one of the top five days of the year," he said, adding that Dec. 26 is the seventh-busiest sales day. Strong gift card sales could push some buying until after the holiday, which could be vital days to the success of the season.

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"We're still looking at 2.4 percent [gain] for the holiday season, even though we're seeing some softness in December," Martin said. But the picture appears bleak for retailers. Many were expecting to post big numbers this past weekend, hoping that shoppers had delayed their buying amid a shorter holiday calendar. Earlier in the season, ShopperTrak had said that Friday, Saturday and Sunday would rank among the busiest days of the shopping season. Weak traffic and tight consumer spending have led many retailers to aggressively discount merchandise and remain open at all hours. In many cases, efforts have led to promotions that matched or exceeded deals on Black Friday weekend. Morgan Stanley analyst Kimberly Greenberger said that half of the retailers she covers offered whole-store discounts, compared with only 35 percent last year. Among those were teen retailer , Express and Ann Taylor, which gave shoppers 50 percent off their entire purchase. (Read more: Ugly sweater? Return policies tighter)

"We think business picked up strongly this past weekend, as anticipated, but total holiday sales are still trending below plan," Greenberger said in a note. "While week three didn't reveal price reductions implying all-out panic, there weren't many positive signs, either." Wells Fargo analyst Paul Lejuez noted that although he thought the weekend seemed busy, it had a lot to make up for—namely, snow and ice that hit the Northeast and Midwest in the prior week, and dramatic price cuts. "We believe weaker merchandise margins will end up being more of an issue than weak sales," Lejuez said. Greenberger sounded a similar note, saying that if sales over the last few days before Christmas aren't terrific, negative fourth-quarter earnings revisions could follow.