The group, which owns Royal Bank of Scotland, NatWest and the Bank of Ulster, apologised to its customers amid reports that they were unable to access their accounts or withdraw money.
The crash comes just months after a computer meltdown that left millions of customers unable to withdraw cash, reports The Telegraph.
People claimed that they had been left stranded, hungry and embarrassed as they were unable to access their own money and had their cards declined.
RBS and its subsidiaries NatWest and the Bank of Ulster issued apologies via Twitter after customers reported problems with cash machines and cards being declined, accessing their accounts online and via telephone.
A statement tweeted by accounts run by all three banks said: “We are aware of the problems our customers are having and apologise, we will provide more information as soon as we have it.”
Stephen Hester, the chief executive of RBS, which is 80% state-owned, was forced to apologise last June after millions of customers were left unable to view an up-to-date balance, move money or pay bills for days after a software update.
The three banks had to extend opening hours to assist customers. A month later more than 700,000 customers were affected by a “human error” that saw some accounts debited twice
Customers took to twitter last night to vent their frustration, with NatWest becoming one of the top topics on the social networking site just over an hour after problems were first reported at around 10pm.
One user, Sharri Morris, tweeted: “Natwest, you left me with no dinner tonight, and left me walking home in the rain! I’d like some compensation please!
Another, Mark Hillman, from Maidenhead, Berkshire, wrote: “Natwest whole system is down! No ATM’s, no online banking and cards WILL be declined. Just found that out the hard way. Pls RT.”
RBS was not immediately available for comment on what was causing the problem
A NatWest customer services adviser said that online and telephone banking, cash withdrawals and payments had been affected for most of the UK’s customers, according to the BBC.
The RBS group has around 17.5 million customers.
Many said that they were considering moving to another bank in light of the most recent fiasco.
Rich Stones asked: “How many chances are #natwest customers gonna give them?”