Marks and Spencer's online orders have been delayed in the Christmas run-up.
The retail chain is telling its website customers that standard deliveries will take up to 10 days, compared with its normal service of up to five days.
The delay follows the popularity of its four-day long promotional event starting on Black Friday.
"Our customer is always our top priority and that is why we've extended some of our delivery options," a spokesperson said.
"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 spokesperson added.
The firm said its service enabling customers to have their online orders delivered to a nominated store was currently taking two days.
The chain said it was reviewing its delivery options for customers on an hourly basis.
The retailer's shares were down 2.8% in early trading making it the biggest faller in the FTSE 100.
IG market analyst Alastair McCaig, Market Analyst, said the fall "highlighted the importance markets are placing on online shopping".
The firm re-launched its website in February, but has faced obstacles since its launch.
All six million customers registered on the old site were forced to re-register, which hit trading in the second quarter of this year.
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