The M&S sale is the second in a fortnight
Marks and Spencer is to hold a second one-day, 20%-off sale on Thursday, as it tries to tempt shoppers to spend despite the continuing economic gloom.
In its second one-day sale in two weeks, M&S said many stores would stay open until midnight to enable customers to take advantage of the discount.
M&S boss Sir Stuart Rose said the firm wanted to give shoppers "a helping hand in the run-up to Christmas".
Debenhams is also offering an "up to 20%-off" sale from Wednesday to Friday.
Sir Stuart said M&S understood that customers were "feeling the pinch".
Thursday's one-day sale will apply to clothing, homewares and wine in both M&S stores and via its website.
The shops will also offer 20% off Christmas cakes and puddings, and the discount comes on top of the recent cut in VAT.
Debenhams has also already held a number of sales days, which it said had increased customer numbers by about two-thirds.
"We're back by popular demand - it's as simple as that," said Debenhams deputy chief executive Michael Sharp.
Online retailer Littlewoods Direct is also cutting prices by up to 60% until 23 December.
"Despite the announced VAT cuts and falling fuel prices, customers are facing tough times this Christmas and we want to help ease them through this period," said Mark Newton-Jones, Littlewoods Direct chief executive.
There are increasing signs of a fall in UK consumer spending, as concerns grow about the health of the economy.
Tesco said earlier this week that the rate of growth in its sales was now the weakest since the early 1990s.
And both Woolworths and MFI have been forced to go into administration, with falling sales being a contributory factor.