According to the Financial Times, it is only the second time in the retailer's history that it has resorted to a pre-Christmas promotion, after current executive chairman Sir Stuart Rose held a similar event in 2004.
The FT claims M&S continued to lose market share in men's and women's clothing, according to TNS figures it has seen.
The company has also had to reshuffle its clothing team due to the departure of George Davies and his daughter Melanie, respectively chairman and director of the Per Una range.
Helen Low, a designer in the womenswear division, has been appointed as Per Una designer and Andrew Skinner, the director of general merchandise planning, has been appointed as Per Una's trading director.
Julian Kilmartin, the trading director of menswear for M&S, is also leaving, reportedly to be replaced by Richard Price, currently head of merchandising for womenswear.
With the 20% discount applying to everything but food, M&S is fighting to attract spenders just as several other retailers enact deep discounts to shift stock.
Debenhams is starting its annual three-day "spectacular", offering 20% to 25% off most items, today. Gap is also running a 12-day 30%-off sale from today and BHS is discounting this week.
Neil Saunders, analyst at retail research firm Verdict, told the Daily Telegraph: "These short, sharp sales are a strategy increasingly used by retailers in the run-up to Christmas.
"All the high street is struggling. John Lewis sales, we know, are poor and everyone else will be in a similar boat.
"Retailers can either sit there and do nothing or they can try and stimulate sales through discounts and promotions."