Sainsbury's saw a record number of orders - one every two seconds - placed for deliveries on the evening its website came back online last Thursday, it claims.
The retailer was forced to shut down its online service last Tuesday and it remained offline for two days. Actual deliveries resumed last Saturday.
But rivals insist they benefited from the hiatus. Ocado said it had seen a spike in traffic of 15%-20% while Sainsbury's was offline. Tesco said its sales increased 5%, while Asda commented it had seen "a significant uplift in orders". It said it expected to retain many of its new customers.
The most likely explanation for the problem was new software, said Andrew Testa, an online expert. He said the software used by Sainsbury's was typically updated every three months. "Before it goes into production it should be tested to see if it would work as expected. But if it has not been tested properly it could take a number of days to get it back up," he explained.
Sainsbury's confirmed 30,000 orders had been lost but said orders were running at normal levels this week. "The system was tested and retested to make 100% sure it would work. We probably could have gone live earlier but wanted to make sure," said a spokeswoman.
"Customers have responded fantastically and have waited for the site to come back rather than look elsewhere."