Banks that left shoppers in Cornwall high and dry on one of the biggest shopping days of the year due to a computer glitch have promised to cover their customers' out-of-pocket expenses.

The technology meltdown hit customers across the UK and left customers of the Royal Bank of Scotland (RBS) Group, which includes NatWest, without the use of their debit or credit cards. It has also affected the banks' websites and smartphone apps.

An RBS Group spokeswoman said customers were starting to access their money again and apologised, "for the system issues that affected our customers".

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.

At Penryn Asda staff were out on the fuel station forecourt warning customers in advance of filling up that there were problems with some cards.

Customers began reporting problems with their accounts around 6.30pm, with some having to abandon trolleys full of food.

The meltdown came on 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.