Major retailers are slashing prices in a pre-Christmas bid to breathe life into the ailing high street.
Marks & Spencer is cutting prices by 20% in its clothing and homewares departments in its biggest one-day promotion for four years.
Retail giants Debenhams and Bhs are also discounting stock this week.
The high street fears the economic downturn will stop people hitting the shops ahead of this year's festive season, with Debenhams saying next month is set to be "the worst Christmas we have ever had".
Analysts say consumers appear to be delaying their Christmas shopping, hoping to catch bargains as shops increase their sales.
M&S began the 24-hour sale online at midnight and will keep a number of large stores open until midnight on Thursday to allow customers to take full advantage of the offers.
M&S spokeswoman Clare Wilkes said: "M&S customers are telling us they are feeling the pinch more than ever and they need some support to help make Christmas extra special."
Asked about speculation that the event would dilute sales leading up to Christmas, she said: "These are really unusual times and it's a very volatile and competitive environment and we need to trade through it.
"We need to compete for every share of the pound that is available to be spent."
Debenhams is in the middle of a three-day £200 million price-cutting campaign across every trading area in all its stores and online.
A Debenhams spokesman said sale customers had queued outside the stores, which were "packed".
Debenhams deputy chief executive Michael Sharp said the retail sector was "staring into the face of the worst Christmas we have ever had".
He said the final weekend before Christmas would be key and that many retailers would have gone on sale by then.
He said: "By the final weekend (before Christmas) everyone will have bought their party dress or coat for the season. That's where to look for mark-downs."
His warning comes as a raft of retailers including Gap, Oasis and Warehouse issue "secret sale" and "friends and family" vouchers.
Analyst Freddie George said: "Christmas 2008 has the feel of being the worst retailing Christmas for many years - more for the higher level of discount activity."