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Keynote reported that Sainsbury's website was unavailable from 12.45pm to 2.10pm BST on Friday 4 July.
Visitors to the third largest UK retailer's website were unable to see the homepage, while its main competitors - including Tesco, Asda, Ocado, Iceland and Waitrose - remained open.
The outage follows a two-day service disruption caused by an internal glitch that is estimated to have cost the retailer £1.5m in lost sales and compensation.
"Following Friday's outage, the homepage was taking approximately 20 seconds to load, in comparison to competitor sites, which were all taking less than four seconds," said Martin Stern, regional manager EMEA at Keynote Systems.