The outage, which left customers struggling to shop for groceries or track orders, began on the morning of Saturday Oct. 23 Saturday and continued into Sunday.
A spokesperson said: “Our online grocery website and app are now back up and running. Our teams have worked around the clock to restore service, and we’re really sorry to our customers for the inconvenience caused.”
The supermarket giant said the problem was due to an attempt to “interfere” with its systems, but added there was “no reason” to believe customer data was affected.
Customers were told they might need to enter a virtual waiting room before accessing the site as Tesco tries to manage the initial flow of traffic.
The “planned” measure is regularly used at peak shopping times to ensure customers have a “smooth experience” on the site, the company added.
The online issue left customers unable to make changes to pre-arranged orders and prompted a flood of queries to Tesco.