Some Morrisons shops are getting back to normal after tech problems affected discounts and online deliveries, the BBC understands.
Customers across the UK were unable to redeem discounts on their loyalty cards and had Christmas deliveries postponed at short notice.
Morrisons is investigating the issues in store and online but has not given any information on the causes of the outage so far.
In response to the problems at the tills, it has applied the loyalty discounts to all customers.
The supermarket is also giving More Card customers an extra 10% off their entire shop today and throughout Tuesday "as an apology."
Customers who shopped on Monday before the 10% discount was introduced can go back in store to redeem the discount, Morrisons said.
Some home deliveries may arrive late, and click and collect customers should wait for an email before going to stores, the company said.
But some customers have told the BBC their online orders - containing their Christmas Day food - have been cancelled altogether.
And others have told the BBC they have experienced similar issues before today, including over the weekend and "in the last couple of months."
Shoppers in Greater Manchester and Leeds have told BBC News that their cards had started to work again.
Morrisons has not confirmed yet if services are restored across the UK, but the BBC understands that many branches are getting back to normal.
'Staff were getting upset'
One customer says he missed out on about £20 of discounts.
"I added everything up as I went around the shop and it came to £70, which with all my card discounts and vouchers should have cost me nothing at the checkout," Steve Weatherby, from Huddersfield, told BBC News.
"But it came up as £90. None of the discounts and offers around the store, like 50p off, had been applied.
"So I had to pay it as I was shopping for Christmas lunch and things we needed."
He added: "The staff were getting upset as they weren't aware of what was happening, and it's not their fault."
Another customer told the BBC her online order was cancelled this morning.
"I booked it ages ago, when the delivery slots first came out for Christmas," said Moira Gray, from Northumberland.
Unable to leave the house due to illness, she added she was "really stressed," as her order included turkey and trifle for Christmas day.
"[Morrisons] said they couldn't do anything... they also said their systems were down," she added, saying that Morrisons offered a £10 goodwill voucher.
Another customer, Lorraine Calvo from Northumberland, had her delivery cancelled this morning and said Morrisons told her this was due to a problem with the IT system.
"I saw the email at 8.30am and was a bit shocked because they text me last night confirming the delivery," she told the BBC.
"It was all my weekly shop to get me through Christmas, and cat food and everything."
Since this story was published online, Morrisons got in touch with Ms Gray and Ms Calvo, and promised to deliver their orders today, free of charge.
It also disputed the claims that these cancellations were due to the main systems issue, telling the BBC that they were separate problems which were dealt with by customer services.
But it did say in a statement: "If for any reason there are isolated cancellations, our team will be in touch directly to resolve them."
'Stuff of nightmares'
Morrisons had been advertising large discounts ahead of Christmas, including some vegetables for 10p.
"There's never a good time for a tech glitch, but for it to happen on what is expected to be the busiest day of the year is the stuff of nightmares," says retail analyst Natalie Berg.
"This will further erode shopper trust and impact profitability."
Today is predicted to be the busiest shopping day for supermarkets this year, according to retail analysts Kantar.
Sales at supermarkets are expected to reach over £13bn for the first December ever.
Morrisons is the fifth-largest market supermarket in the UK, according to Kantar, with 8.6% of the grocery market in the 12 weeks leading up to 1 December 2024.
"Retailers are putting more and more deals through their loyalty cards and if shoppers can't access them, they will feel cheated," said Ronan Hegarty, news editor of trade publication The Grocer.
The Morrisons website was also down for a period in the morning, with error messages on some pages suggesting invalid or late responses from servers.