Retail data company Springboard said footfall for the first three hours of Dec. 26 was down 10.5 percent year-on-year.
Many large retailers launch sales offering big discounts on Boxing Day to try to entice domestic and foreign consumers to engage in a second wave of buying after Christmas.
But Springboard said many shoppers would be making their purchases online.
"The scale of the drop this year falls outside of an ongoing retail trend," it said. "The double impact of early discounting and online shopping was clearly seen to negatively affect pre-Christmas footfall trends and it seems their influence cannot be ignored in relation to Boxing Day."
Diane Wehrle, the firm's retail insights director, said Christmas Eve footfall had also fallen 5.2 percent year-on-year."