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Marks & Spencer and John Lewis are among major names to start discounting online in the hope that shoppers will be browsing sites over Christmas.
Sales online have traditionally begun on Christmas Day or Boxing Day.
Reports that millions of consumers will spend the holiday shopping online prompted a warning from Church leaders.
Former Archbishop of Canterbury Lord Carey said Christmas was a "special time" and should be spent with family and not logging-on. "We are now in danger of the gadgets taking over our lives and we are not in control of them," he said.
And Steve Jenkins, a spokesman for the Church of England, urged people to make time to go to church and "maybe spend a bit of time online spending their new Christmas vouchers".
But with the British Retail Consortium (BRC) warning that Christmas sales generally were likely to be "acceptable" rather than "exceptional", retailers are looking for every opportunity to maximise sales.
M&S began its sales online at midday on Monday, while department store John Lewis said it would cut online prices when its stores close at 1700 GMT.
Debenhams has already started its online sale. Online giant Amazon will start its sale on Christmas morning, a day earlier than usual.
Continue reading the main storyWe suspect that people will likely be more careful in buying - or reluctant to buy - items that they don't really want or need in the sales”End Quote Howard Archer IHS Global Insight A report from Ofcom, the telecoms regulator, has estimated that shoppers spend an average £1,000 a year online each year. This is more than in any other country, including the US.
The popularity of online retailing contrasts with continued problems for the High Street.
'Modest'The BRC forecast that £5bn would be spent in the shops on Saturday and Sunday combined, the last weekend before Christmas. But Richard Dodd, the BRC's head of Media and Campaigns, said that was nothing to get excited about, adding: "It's been a very busy weekend which will be crucial to delivering a Christmas that is acceptable, rather than exceptional."
He forecast a modest increase in cash spending on a year go, but not necessarily any significant increase because household finances are under pressure.
Economist analyst Howard Archer at IHS Global Insight said the weakening in household finances could not come at a worse time for retailers, and it highlighted why Christmas spending was so modest.
"The suspicion has to be that consumers will be especially keen to take advantage of genuine major bargains in the sales to acquire items that they cannot otherwise afford or are reluctant to make at the moment," he said.
"However, we suspect that people will likely to be more careful in buying - or reluctant to buy - items that they don't really want or need in the sales."
Nevertheless, some shops reported brisk trading.
Sainsbury's reported its busiest ever hour in terms of customers served from midday to 1pm on Sunday, while 35 branches opened at midnight and traded until 6pm on Monday.
More than a million visitors were expected in London's West End during the three-day period from Saturday to Christmas Eve, during which more than £100m was expected to be spent.
'Critical condition'Bluewater shopping centre in Kent was also anticipating a surge in sales on Monday as Saturday's footfall was up 14% from the previous week.
And the problems facing retailers was underlined on Monday in a report by business recovery group Begbies Traynor. It estimated that tough Christmas trading conditions had left nearly 140 firms in a "critical" condition.
Book retailers were among those in significant distress, hit by competition from players such as Amazon, while convenience stores have suffered from the rising dominance of supermarkets.
However, online retailers have seen sales figures improve, Begbies said.