Both Sainsbury's and Waitrose suffered website failures yesterday, leaving hundreds of customers without their Christmas shopping.
Shoppers vented their rage on Twitter following the problems caused by the outages, with Sainsbury's allegedly cancelling hundreds of customers' orders by accident following a computer failure, according to the Guardian.
Another issue affected Waitrose.com for four hours yesterday, resulting in orders not being delivered last night and Monday morning.
Sainsbury's told the Guardian that its website was down for half an hour at 10.50pm on Sunday evening, but one customer told IT Pro's sister title PC Pro that the timing affected customers trying to order in time for the 11pm deadline for Christmas shopping.
Customers were met with a holding message advising them to contact the retailer by phone. However, Ian Moyse, sales director at Workbooks - and one of the customers affected by the issues - said the lines were too busy and kept directing people back to the website.
He added: "Did they not expect a peak load at Christmas? The site was creaking for hours during Sunday and then fell over just at the important time that many were trying to checkout their carts to reach the 11pm deadline for Christmas deliveries."
While customers of both stores have been offered alternative delivery dates, some come after Christmas Day.
Moyse said he has not been contacted following a complaint he made.
Moyse claimed it demonstrated the consequences for businesses whose website infrastructure cannot support large demand, after wine merchant Oddbins changed IT providers after similar issues with its own website.
"This is a PR disaster for them which IT will have to answer for," said Moyse. "This shows where businesses online still have not all architected for scaleability and contingency.
"With today's technology there is no excuse, elasticity can be built in using cloud solutions to avoid any such overload and yet such a big brand has failed its customers so abysmally."
The news comes after Black Friday in the UK caused retailers' websites to crash under huge demand last month.
A Sainsbury's spokesperson told the BBC: "We experienced a brief technical issue with our website last night, which has now been fixed.
"Some customers experienced difficulties with booking or amending their delivery slot. We're very sorry for the inconvenience caused."
A Waitrose spokeswoman told IT Pro: "The temporary IT problem was for four hours yesterday (Sunday morning) - it is now fine and we have contacted any customers affected to apologise and rearrange delivery.
"No orders have been cancelled. The issue was with the system that our shops use for picking orders for delivery."
IT Pro contacted both supermarkets for comment but they had not responded at the time of publication.
This article was originally published at 3.40pm on December 22, but was updated at 4.30pm to include Waitrose's statement
