The retail giant said there had been attempts “to interfere with our systems” which had caused an issue with the search function on the site.
Customers noticed on Saturday that they were unable to order goods or track deliveries on the website.
On Sunday, Tesco said there was deliberate disruption but reassured customers that their data was safe.
A Tesco spokesperson said: “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.
“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.”
Shoppers had complained about a lack of information, with many wanting to know how to cancel orders and whether they would get money back.
Tesco’s online sales have been boosted by the pandemic and provided the retailer with strong profits.