Tesco, Marks and Spencer, Currys, Argos, Boots, House of Fraser, Debenhams and others are offering up to 50 per cent off items.
Richard Brasher, Tesco's commercial director, said: "Customers are telling us that they are delaying their main Christmas purchases as they wait for bargains. Some say that a bit of the pleasure they usually get from buying gifts has gone in the current climate.
"Our Half Price Sale reassures customers that we're not making them wait."
Analysts said they were alarmed by the scale of the sale.
Jason Gordon, retail partner at consultancy Ernst & Young, said: "This is the clearest indication yet of quite how difficult life is on the high street. For the country's largest retailer to launch such a significant sale really is amazing."