The grocer took just over 48 hours to get it back up after discovering a problem with the processing of orders on Tuesday, June 17.
It declined to comment on the reason for the failure or the measures it had to take to get the site back online.
Various estimates put the number of customers affected at between 20,000 and 30,000. They were offered a£10 voucher by way of compensation.
Although the site was back up by Thursday evening, the earliest that customers were able to book a delivery for was Saturday.
Sainsbury’s loss appears to have been Tesco’s gain. Hitwise reported that 8.36 per cent of visitors to Sainsbury’s main home page went next to the Tesco web site on Wednesday, June 18, when the grocery ordering site was down all day.
In comparison, the percentage of customers leaving Sainsbury’s site for Tesco for the whole of the previous week was 1.38 per cent.
Forrester senior analyst Victoria Bracewell Lewis, who specialises in e-business, told Retail Week that although UK grocery shoppers were fairly loyal, it was unacceptable for a grocer the size of Sainsbury’s to have a single point of failure in its systems and have no back-up available.
However, she added that Sainsbury’s was right to keep the site down for thorough testing rather than risk a backlash due to repeated problems.