Bank divisions of The Royal Bank of Scotland Group suffered from a technology problem beginning late Wednesday night, preventing customers from accessing online banking, making card payments, or withdrawing cash for a few hours. The outage affected customers at RBS (RBS), NatWest and Ulster Bank. The three banks have 17 million personal banking customers.

RBS blamed the failure on a "hardware fault" that blocked customers' access to online banking, ATM use and point-of-sale payments from roughly 9 p.m. to 11 p.m. Wednesday.

"We apologize for the disruption our customers experienced last night. All systems are up and running as normal, though any customers with any individual problems should get in touch with us," said RBS in a statement. Requests for more information about the nature of the breakdown were not responded to immediately.

The slowdown follows a major software meltdown that affected RBS, NatWest and Ulster Bank customers from accessing their money in June 2012.

RBS said the outage Wednesday was not related to the mishap last summer. "It was much easier to fix, though clearly an unacceptable failure," the banking group said in a statement.

Customers vented their ire with the banks on social-networking sites like Twitter and Facebook. Dan Bratton (@danbratton) tweeted Wednesday: "If you bank with Natwest I feel bad for your son …You got 99 pounds and you can't withdraw one."

Bratton's Tweet was retweeted 192 times as of Thursday morning.

Anton Sutton (@anton_sutton) tweeted, "Natwest how hard is it to put a notice on your website to say you've got service issues and online banking is borked? Found out via BBCnews."

Meanwhile, more than 200 comments flooded NatWest's Facebook page from angry customers. One of them was from Myles Roberts: "BBC news are telling people the problems been fixed. I'm on the phone in Radio 4 now telling them that you're lying. It's not been fixed. I can't withdraw cash, I can't log in to my online banking on either a PC, Mac or iPad."

All three brands — RBS, NatWest and Ulster Bank — responded to the criticism on their social media sites. The banks apologized to their customers and encouraged those experiencing any lingering banking troubles to connect with them. For example, one tweet from the RBS Group reads: "Any customer who was left out of pocket due to this outage should get in touch so we can put things right for them."

Communicating to customers through social-networking sites during a disaster is crucial for banks worldwide, say analysts.

"It's very important for a bank in this case to communicate a lot with their customers, otherwise their customers will communicate on their behalf," says David Albertazzi, senior analyst with Aite Group. "Communicating is key in social networks. … It's an opportunity to be close to their customers.

"I think [RBS divisions] are doing it right. …They are addressing issues where their customers are."

The hardware glitch suggests a problem with RBS's core systems, he says. "Any downtime is not good for a financial institution," he says.

Timing can make it worse. When the outage occurred last night, many people may have been out dining out and unable to pay their bills.

American banks have suffered from website outages in recent months, too. Like the RBS Group incident, those outages ignited customers to vent their banking frustrations online.

Looking ahead, Albertazzi said it's important for banks to further communicate with customers who suffered from its hardware issue. A bank could identity impacted individuals by looking at the logins of failed online banking sign-ons, for example, during its post-mortem analysis, he says. Banks should follow up with those customers who were potentially embarrassed by their inability to pay for something, he advises.

"Social media is used early on to communicate to everybody and doesn't distinguish between those who are affected and those who aren't," Albertazzi says.