Users of the Tesco website and app are experiencing disruption following an "attempt to interfere" with the system, the retail giant has said.
The website and app have been hit with outages since yesterday morning, meaning customers cannot place or track orders.
Website users are given an error message when they try to access groceries or use the search bar to look up products.
It is unclear how widespread the issues are or how many customers have been impacted.
A Tesco spokesperson said: "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."
Tesco first referenced the problem on Saturday morning saying there was an "issue" and that they were "working hard to get things back up and running".
Sunday was the first time they mentioned an interference attempt as being behind the outage.
Customers have complained to the retailer on Twitter, with many wanting to know how to cancel orders and whether they can get a refund.
One posted: "Tried to DM [direct message] to cancel my order for today last night but nobody has responded, and website and app still down! I presume I can just turn away the order when it arrives? There are only two bottles of alcohol in there to hold the slot so it's completely useless to me."
Another said: "Any updates when it might be working again?? I have a delivery booked for tomorrow and need to update order as only ordered random items! I do not want to cancel as it is hard enough to get a slot, just want to add the things I need. Its now over 24 hours have you been hacked?"
The issues come after Tesco saw online sales soar during the pandemic and lockdown.
These rose by 77% to £6.3bn between February 2020 and 2021, growing to represent nearly a fifth of sales over the winter period.
Weekly delivery slots more than doubled to 1.5 million over the 12 months.