Furious NatWest customers have deluged the bank with complaints after an IT hitch blocked them from accessing their cash for the second time in nine months.

The problems, which also affected sister firms Royal Bank of Scotland (RBS) and Ulster Bank, follow a huge computer meltdown last summer which cost parent company RBS Group £175 million to put right.

Customers started reporting problems at around 9pm on Wednesday night, which meant that they were barred from accessing money at ATMs, with reports of some having their cards "swallowed" at cashpoints. Online and telephone banking services were also disrupted.

The bank apologised for the "unacceptable failure" and said the issue was resolved by around 1am, although some customers complained on NatWest's website that they were still having problems.

Others also vented their annoyance that they had stuck with the bank only to be hit with the second IT collapse in less than a year. A section of NatWest's website where people can suggest ideas was swamped by a string of complaints, some of which referred to the bank's "helpful banking" slogan.

One customer wrote today: "I can't get to work as card declined on train, so lost a day's wage and (you) expect me to pay to call for info!"

Another 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..."

It was unclear exactly how many of the 17 million personal banking customers of NatWest, RBS and Ulster Bank had been affected by the problems. The group said that they were caused by a "hardware fault" which was unrelated to last summer's meltdown and it promised that customers will not be left 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."

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."