The banking group behind NatWest's latest IT collapse has promised to compensate customers who were left out of pocket as a result of being blocked from accessing their cash.

Furious customers have deluged the bank with complaints about its second computer hitch to happen within a year, which has also affected sister banks Royal Bank of Scotland (RBS) and Ulster Bank.

RBS Group has apologised for the "unacceptable failure", which it said was due to a hardware fault unrelated to last summer's meltdown that cost the group £175 million to put right.

Problems from around 9pm yesterday meant that customers were barred from withdrawing money at ATMs, with reports of some having their cards "swallowed" at cashpoints. Online and telephone banking services were also disrupted.

The bank said that the hitch was cleared up a couple of hours later, but some customers were still reporting problems on NatWest's website today.

The group is urging customers who have been left worse off to get in touch and promised that they will not be permanently out of pocket.

A statement from RBS Group said: "This problem was caused by a hardware fault and was not related to the issues we experienced last summer. It was much easier to fix, though clearly an unacceptable failure.

"Any customer who was left out of pocket due to this outage should get in touch so we can put things right for them."

A spokeswoman for the group said claims will be looked at on a "case by case basis" to work out if people should be compensated for any extra costs.

Customers will not be automatically entitled to a payout just because they had problems and those who believe they are entitled to some cash should keep evidence such as receipts.

The BBC quoted NatWest customer Darren Reuben who said he had to get someone else to cover a restaurant bill yesterday evening. He said that he was given £70 in compensation to help cover the embarrassment caused after turning down an offer of £30.

"It was paid into my account straight away," he told the BBC. "While it was embarrassing, they did me a good turn."

It is unclear exactly how many of the 17 million personal banking customers of NatWest, RBS and Ulster Bank had been affected by the problems, although many people vented their frustration online.

One Twitter user said: "Natwest, you left me with no dinner tonight, and left me walking home in the rain!"

NatWest's own website was swamped with angry messages from customers, with several complaining about a lack of information.

One customer wrote: "Tell us what's happening! You put up all sorts of irrelevant guff on the online banking service, which we have to click to accept - but not what we really want to know, the status of your banking systems."

Other complaints referred to NatWest's "helpful banking" slogan.

Another customer said: "Really upset that NatWest comms have failed yet again. We understand that things sometimes go wrong - but 'helpful banking' is all about helping people and communicating with them. After last year's fiasco I stuck with you, but now I'm not going to..."

Someone else reported on the website that they had spent half an hour in a call-waiting queue after having their card declined "before giving up".

In its recently-released annual report, RBS said it had been investing in technology to make it easier for people to bank with the group, including introducing more than 200 ATMs and cash deposit machines.

Stephen Hester, the chief executive of RBS, which is 80% state-owned, apologised last summer after millions of customers were left unable to view an up-to-date balance following a software update.

The group took a £175 million hit as a result of last summer's IT chaos which left NatWest, Royal Bank of Scotland and Ulster Bank customers locked out of their accounts.

The annual report showed that the bulk of the redress costs related to Ulster Bank, where the problems took weeks longer to clear up, with the bill reaching around £82 million.

Which? executive director Richard Lloyd said: "Customers will be rightly angry at another loss of essential banking services.

"These problems yet again raise wider questions about how robust and up-to-date banks' systems are.

"The banking industry has been allowed for too long by regulators to rely on sticking plaster to keep antiquated systems going rather than invest in modern and robust technology that is needed to give consumers confidence that they can always access their money - or move accounts - when they need to."