"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," a Tesco spokesperson said.
The grocer first experienced the problem on Saturday with customers unable to order goods and track deliveries.
"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," a Tesco spokesperson said.
It said there was no reason to believe that customer data had been compromised.
(Reporting by Andy Bruce and Akriti Sharma in Bengaluru; Editing by Raissa Kasolowsky and Stephen Coates)