The outage, which left customers struggling to shop for groceries or track orders, began on Saturday morning and continued into Sunday.
The supermarket giant said the problem was due to an attempt to “interfere” with its systems but there is “no reason” to believe customer data has been affected.
A Tesco spokesman said on Sunday: “Since yesterday, we’ve been experiencing disruption to our online grocery website and app.
“An attempt was made to interfere with our systems. which has caused problems with the search function on the site.
“We’re working hard to fully restore all services and apologise for the inconvenience.
“There is no reason to believe that this issue impacts customer data and we continue to take ongoing action to make sure all data stays safe.”
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Tesco posted a tweet on Saturday which told customers: “We’re experiencing an issue with our website and app and are working hard to get things back up and running. We apologise for any inconvenience.”
By Sunday lunchtime online shoppers were met with a message which said: “Sorry, there’s currently an issue on our website.
“We’re busy fixing it right now and hope this doesn’t cause you too much inconvenience. Please come back and try again later.
“Thank you for your patience.”Internet Explorer Channel Network