NatWest has apologised to customers after its online banking and mobile app services went down during the Black Friday sales.
The Royal Bank of Scotland (RBS) Group, which owns NatWest, has said it is unsure what has caused the problem but is aiming to get its services back up and running.
NatWest customers have complained on social media, claiming they have been unable to make the most of Black Friday offers.
Others have said they have been unable to pay their bills.
The bank's online service for business finances, Bankline, is also down.
Brandon Leigh, from south Manchester, said the NatWest failure has impacted his business.
Mr Leigh runs a first-aid training business and planned to buy 36 mannequins and equipment at a 20% Black Friday discount.
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The items would usually retail for a total of £5,000.
Our Online Banking, mobile app and Bankline are currently facing intermittent problems. We're working hard getting them back up and running smoothly for you.
Well post an update when we have more information. And remember to contact us if you need urgent help.
— NatWest (@NatWest_Help) November 29, 2019
He told Sky News: "For me, it's a big saving. We went to the supplier but I can't pay them. It's terrible, I need them.
"I rang NatWest and was held in a queue for ages. They said they are working on the problems and it would be sorted, but not in the next hour or two. I can't get in the mobile app, or internet banking."
He added: "Hopefully my supplier will honour the price for me next week."
One Twitter user, named only as Siobhan, wrote: "Well that's just great, pay day, Black Friday and #natwest online is down. @NatWest_Help #BlackFridaySale #timetochangebanks."
A Twitter user called Luce wrote: "Black Friday and NatWest mobile banking is down. Never known frustration like it."
Bolaji Lawal-Sofoluwe said: "I am trying to pay a bill to get goods cleared.
"NatWest is becoming so problematic."