Record numbers visited UK retail websites on Boxing Day, with analysts suggesting shoppers are also using the internet to identify bargains.
Information service Experian said UK consumers made 113 million visits to retailers' websites during 26 December.
The number of visitors to the High Street on the same day was up 0.64% on last year's Boxing Day trade, according to Experian.
Some big name retailers started their online sales on Christmas Day.
UK internet users made 84 million visits to retail websites on Christmas Eve and 107 million visits on Christmas Day, up 86% and 71% respectively compared to the same days in December 2011, according to Experian.
The Boxing Day level - 113 million visits - was 17% up on the same day in 2011. Typically, during the year, there is an average of about 70 million visits on Mondays - the busiest day of the week for online shopping.
"The UK sales creep continues to advance so that now the post-Christmas sales are starting before Christmas," said James Murray, digital insight manager at Experian.
"Five years ago we called it the January sales, before it became the Boxing Day sales, now retailers have to call it the winter sales as discounting starts earlier to encourage higher spending."
Shoppers headed back to the High Street on Thursday, with large department stores such as John Lewis throwing open their doors for clearance sales.
Yet, retail consultants have said that many people heading out to the shops will have already browsed online to choose the items they want.
The squeeze on family finances is likely to keep the lid on retail sales, especially on big ticket items.
A lack of activity in the housing market is also reducing demand for some household items that might have been replaced as people move home.
However, some positive news in employment levels means that some stores could still record a decent level of sales in the significant post-Christmas sales period.
Experian Footfall said that there was "quiet optimism" on the High Street with the number of shoppers up slightly on 26 December, compared with the same day in 2011.
The growth of the internet means that the peak in sales might already have taken place.
Mr Murray, of Experian, said that 26 December was traditionally the single biggest shopping day of the year online.
And now, shoppers are using digital devices such as tablets and smartphones to search for bargains - then only travel to those specific shops to buy those items.
"The internet has been a huge factor in retail all year, and has an influence on the High Street with shoppers doing more research beforehand," said Matt Piner, founder of retail research agency Conlumino.
He said items such as laptops and furniture in particular were identified by shoppers during online browsing, rather than in a store.
John Lewis, which starts its sale in department stores on Thursday, said it had seen notable activity during its online clearance sale. That started at 1700GMT on 24 December.
On Christmas Day, the department store said online sales peaked late in the evening. Items that proved popular included electrical items, sheets and pillowcases, luxury towels and candles.
Analysts said the departure of some high-profile names from the High Street had helped some of the remaining department stores. However, many had targeted "cautious" shoppers with discounts in the run-up to Christmas, according to Rahul Sharma, of Neev Capital, a retail consultancy.
He said that shoppers were offered discounts of 20% to 30% in the build up to Christmas, to tempt them into buying items for themselves, as well as presents.
This meant that clearance sales might be muted this year, with many of the items that stores wanted to shift already having been sold.
Analysts have suggested that DIY and gardening will see the strongest performance in the retail sector in 2013, compared with 2012.
Poor weather in the past 12 months meant that sales have been low. This, together with homeowners improving homes ready to go on the market, should lead to a rebound in the coming year, according to Verdict and SAS UK.
The groups predicted that spending on food was likely to raise roughly in line with inflation.
However, they say that music and video spending will be hit the hardest, with a predicted 6.3% fall compared with 2012, owing to online streaming and cheaper internet prices.
The amount people spent online was expected to account for 12% of total retail spending, they added.
"UK retailing is set for another year of tough trading," said Maureen Hinton, of Verdict.