Marks & Spencer, Britain's biggest clothing retailer by sales, admitted today it had been forced to extend delivery times for online orders as a new distribution centre has struggled to cope with heavy Christmas demand. 

Improving its online offer is a central plank of M&S's strategy to reverse nearly a decade of market share decline in its clothing business.

Its stock was the second-biggest faller on London's FTSE 100 index of blue chips today, falling by over 3%. 

The retailer had attracted a jump in orders during its four-day promotion, which started on November 28 to coincide with "Black Friday" events across the industry. 

To ensure delivery options offered to customers were accurate, M&S extended slots on its website for standard delivery from three to five days to as many as 10 days and withdrew next-day delivery, prompting some complaints. 

Next-day delivery to homes has since been switched back on, but deliveries to stores are still taking two to three days. 

"Our customer is always our top priority and that is why we've extended some of our delivery options," M&S said in a statement today. 

"The vast majority of orders are delivered on time. If we do miss a delivery date, we will do all we can to rectify it for the customer," the company added. 

The retailer's fully automated 900,000 square foot, e-commerce distribution centre in Castle Donington in central England opened in April last year and was scheduled to reach full capacity in the run-up to Christmas this year. 

"It's the first Christmas of having all M& there. With an operation of that scale, you are always going to get some onsite challenges," an M&S spokesman said. 

M&S's new website went live in April this year but has also been beset by teething problems. 

M& sales fell 6.3% in the first half to September 27, though the firm said last month it was on track to return to growth ahead of the peak trading period. 

Some small improvements in women's clothing helped M&S post its first rise in first-half profit for four years in November ,though analysts said it was too soon to declare a turnaround.