Complaints about the late arrival of online deliveries have risen by a third in the run-up to Christmas, according to a government advice service.
Saturday was "way beyond expectations" on Oxford Street
Consumer Direct says it has received 3,000 complaints in six weeks.
Royal Mail says the "vast bulk" of Christmas post will arrive on time and online retailer Amazon says its delivery success rate is above 99%.
Meanwhile, retailers reported Saturday was their busiest day of the year with sales exceeding expectations for many.
The UK's 27 million online shoppers are expected to have spent £15bn online in the run-up to Christmas Day - up 60% on last year, according to Interactive Media and Retail Group (IMRG), the industry body for the electronic retail community.
Online retailers use a variety of courier companies to make deliveries to customers, as well as Royal Mail.
Amazon says it has been dispatching 750,000 parcels a day in the run-up to Christmas.
Royal Mail has predicted that it will deliver a record 120 million items ordered over the internet this festive season - double the number handled three years ago.
In total, it expects to deliver some two billion items of Christmas post.
Royal Mail's workforce has been boosted by an extra 20,000 members of staff in the four week run-up to Christmas, with more than 2,000 staff taken off non-operational duties to help with deliveries.
A spokesman said: "Royal Mail has already delivered the vast bulk of this year's Christmas mail, and today we're pulling out all the stops with the aim of ensuring that all remaining items posted before recommended last posting dates are delivered in time for Christmas Day."
Tim Danahar, editor of Retail Week magazine, told BBC News the real problem with online shopping had become the final part of the delivery process, involving Royal Mail and the private carriers.
He said: "They've really struggled just with the sheer volume of parcels they've had to deliver."
He added: "It's not hard to see it coming. Internet shopping has grown at an extraordinary rate every year for the past few years.
"It's about confidence. People are leaving it later and later now because if you have a good experience you think, 'well, I'll leave until a week before Christmas' and that's where these mountains of parcels have really built up."
High street retailers who had feared shoppers were planning a frugal Christmas after a slow start to December are relieved after a busy weekend in the shops.
On Saturday, Bluewater shopping centre in Kent said it had been packed, with car parks full throughout the day. The Boots chain said it had recorded its busiest weekend of all time at the centre.
Brent Cross in London said it expected to have had 200,000 shoppers over the "three-day weekend" by the end of Christmas Eve.
Tom Nathan, the centre's general manager, said: "With Christmas Eve falling on a Monday, shoppers are taking full advantage of the weekend to finish buying all their presents.
"We expect that the Christmas Eve afternoon rush will be driven mainly by food shoppers and younger men, who typically are the ones most likely to leave their Christmas shopping to the last minute."
The New West End company which represents retailers on London's Oxford Street, Regent Street and Bond Street, said Saturday had been "way beyond expectations", though Sunday was not so busy.
British Retail Consortium director Kevin Hawkins said there would be clear differences between retail's winners and losers this year.
He said: "Those that got it right on price and availability should have done well."