Marks & Spencer's online sales suffered in December, despite other high street retailers reporting e-commerce success over the period.
The company's third-quarter financial report revealed a decline of 5.9 percent in sales made through its M&S.com online store compared with the previous year.
Rival retailer John Lewis, on the other hand, saw a hike of 5.8 percent driven in part by huge Black Friday sales.
M&S enjoyed similar success on Black Friday, but the sales rush resulted in a backlog of orders that affected the retailer's online performance during the Christmas period.
The disruption came from M&S' automated national distribution centre in Castle Donington, Leicester, where the backlog led to extended delivery times of up to 10 days.
An M&S spokesman told V3 that, while the company has addressed the problems at the centre, it had a direct effect on online sales.
"Disruption at our distribution centre in Castle Donington strongly impacted M&S.com and in turn [general merchandise] performance in December," he said.
M&S was forced to hold back on the discounting of products, which then had an adverse effect on sales in-store and online.
The firm declined to speculate on whether the longer waiting times caused would-be customers to shun M&S.com for rival online retailers.
The decrease in online sales could also be attributed to the decision in May to migrate the M&S.com web platform from Amazon to its own in-house servers, which M&S said at the time would take "four to six months" to settle in.
Marc Bolland, M&S chief executive, said in June that the settling-in period had caused online sales to decline in the company's first quarter.
"As anticipated the settling in of the new M&S.com site has had an impact on sales," he added.
M&S may have been hoping for better online performance over the Christmas period, but remains confident that the company "remains on track to return to growth ahead of our peak trading period".
Online retailers are now outperforming old high street brands and M&S will need to ensure that the e-commerce side of its business is up to the standards of its rivals if it wishes to remain competitive.
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