A banking glitch on one of the busiest online shopping days of the year left millions of shoppers unable to pay for transactions using their credit or debit cards.
The technology meltdown has left customers of the Royal Bank of Scotland (RBS) Group, which includes NatWest and Ulster Bank, also affected the banks' websites and smartphone apps.
An RBS Group spokeswoman said some customers were starting to access their money again.
"We are very sorry for the system issues that affected our customers this evening," she said.
"Our customers are reporting that services are coming back on line. We will confirm when all systems have returned to normal service.
"If customers have been left out of pocket as a result of these system problems, we will put this right."
The spokeswoman did not reveal what caused the glitch, but said the company would compensate customers for it.
"If any customer is unable to resolve an issue caused by the disruption, they should get in touch with our call centres or come into a branch in the morning where our staff will be ready to help," she said.
But angry customers venting on social media were still reporting problems when the spokeswoman said they were beginning to be resolved, with some detailing distress and embarrassment when they were unable to pay for items like petrol.
Durham woman Nickie Gott tweeted: "Thanks #natwest for leaving me with an unpleasant petrol attendant when my business and personal card were rejected for fuel."
Customers began reporting problems with their accounts around 6.30pm, with many having to abandon trolleys full of food or left unable to pay for dinner.
When the problem first arose, the spokeswoman said: "We are aware of the technical issues being experienced by customers and are working to get them fixed urgently.
"We apologise to customers for the inconvenience caused."
The meltdown came amid a day dubbed Cyber Monday, which was typically when the highest amount of online transactions were expected so shoppers could meet pre-Christmas delivery deadlines.
Twitter was inundated with customers sharing awkward situations the problem left them in.
"Natwest is down. Just looked like a broke alcoholic attempting to buy booze, but apparently it is not my fault. Thanks Natwest," Holly Larsen, from Hither Green in south-east London tweeted.
Liz Stinchcombe wrote: "Natwest gone down again. Brilliant especially when your in your natwest uniform doing your food shop."
Others seemed relieved to understand why their accounts were being declined.
"So that's why my card got declined 3 times and I couldn't log in to online banking," London woman Charley Francis wrote.
Meanwhile, it was reported on Monday that Royal Bank of Scotland bankers are set to rake in £500m in bonuses this year, despite the state-backed bank suffering a series of scandals like allegations that it was "killing off" small businesses on purpose in order to seize their property.