Marks & Spencer will hold a one-day 20% off sale on Thursday amid further signs of anxiety over the downturn in consumer spending.
The retailer will slash prices in its fashion and homewares departments in its biggest one-day promotion for four years.
M&S will join other big name high street retailers discounting their stock this week, including Debenhams and Bhs.
It came as a leading firm of stockbrokers made a further cut to its profits forecast for the retailer. Seymour Pierce stockbrokers downgraded its 2008/9 estimate from £650 million to £630 million, but left the following year's forecast unchanged at £535 million.
Analyst Freddie George said the M&S sale was a "clear sign that sales are well behind budget" as it enters the festive season.
He said consumers appear to be delaying their Christmas shopping, hoping to catch bargains as shops increase their sales.
"Christmas 2008 has the feel of being the worst retailing Christmas for many years - more for the higher level of discount activity," he said.
M&S executive chairman Sir Stuart Rose has not attempted such a large one-day sale since his "Christmas Spectacular" just after he took over in 2004.
A source said the earlier sale was "for very different reasons".
M&S has recently struggled as shoppers snub its premium end food offering, while its clothing is also being squeezed by competition at both ends of the market.
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