LONDON, United Kingdom — Marks & Spencer Group Plc shares fell the most in more than two months after the U.K.'s largest clothing retailer experienced delays delivering some customers' online orders at the busiest time of the year.

M&S today confirmed reports that its Castle Donington distribution center in central England is struggling to process a surge of online orders prompted by the Black Friday weekend. The stock was down 3.1 percent at 1:38 p.m. in London.

The “vast majority” of orders are being delivered on time, spokesman Daniel Himsworth said in an e-mail.

The news adds to concern that the company’s strategy to consolidate 110 warehouses nationwide into six distribution centers will undermine attempts to restore online sales growth, which Chief Executive Officer Marc Bolland told investors on Nov. 5 would occur by the end of last month.

February’s introduction of a redesigned website has weighed on Bolland’s attempts to turn around M&S’s clothing division after three years of declining same-store sales. The retailer’s upscale food division has performed better amid a grocery price war that has squeezed the profitability of big supermarkets from Tesco Plc to Wm Morrison Supermarkets Plc.

Marks & Spencer customers are not currently able to place in-store click-and-collect orders for the next day, while standard deliveries to home addresses, normally promised within five days, are now guided to take as long as 10.

“Our customer is always our top priority and that is why we’ve extended some of our delivery options,” Himsworth said.

The delays have led to a deluge of complaints from customers on the retailer’s Facebook page. The company said it will still offer next-day delivery for 3.99 pounds ($6.24).

By Tom Beardsworth; editors: Celeste Perri, Paul Jarvis, Thomas Mulier.