RBS, NatWest and Ulster Bank say the problems that meant customers could not access their bank accounts online on Friday have been "resolved".
For about five hours customers were locked out of their accounts until the problem was fixed mid-morning.
Many took to social media to complain, a day after technical problems also hit rival Barclays.
An RBS spokesman said: "We would like to apologise to customers who experienced issues this morning."
He added that: "The issue has now been resolved and no customer will be left out of pocket as a result of this."
RBS Group - which owns the RBS, NatWest and Ulster Bank brands - said the failure was caused by a "technical glitch" in a regular update to their firewall which was applied early on Friday morning.
After the problems appeared the update was reversed, allowing customers to get into their accounts once again. The banks' ATM network and telephone banking was not impacted at any stage.
The bank emphasised that it was an "access issue" and there is no evidence that customer data was compromised.
RBS chief executive Ross McEwan apologised this morning.
According to the group's latest annual report it has 19 million customers in the UK and Republic of Ireland with 5.5 million active mobile app users.
Many of them shared their frustration on social media, with a number pointing out that the problems arrived at a terrible time - on payday.
No #natwest online banking on pay day. Really helpful guys. Sort it out ASAP please. I have bills to pay! 😡— Pamela Brewer (@Happymool) September 21, 2018
Customer Paul Murphy told the BBC: "This is just what you need as the weekend approaches and bills to pay."
Jess Cochrane said: "It's payday, I can't transfer my wage to the joint account all the bills come out of, I have no card and no branch near me."
'Not good enough'
The issue was just the latest in a growing series of problems that customers of Britain's banks have experienced in the last 18 months.
The RBS group had a similar problem in April last year, when its banking apps stopped working.
In 2014 it was fined £56m by regulators after a 2012 software issue left millions of customers unable to access accounts for weeks.
Earlier this year TSB's huge IT meltdown led to weeks of pain for customers and the eventual resignation of chief executive Paul Pester.
But this week has marked three bank outages.
Customers of online challenger bank Cashplus - which targets people with poor credit histories - were unable to access their accounts, make cash withdrawals, or make or receive payments earlier this week.
Then on Thursday Barclays customers were locked out of their accounts online for several hours. The bank said it is yet to discover what caused the problem.
A Barclays spokeswoman told the BBC: "Our priority yesterday was to get services back online for customers. We're currently investigating the root cause of the outage."
Nicky Morgan MP, Chair of the Treasury Committee, said: "It simply isn't good enough to expose customers to IT failures, including delays in paying bills and an inability to access their own money.
"High street banks justify the closure of their branch networks on the basis that they are providing a seamless online and mobile phone banking service. These justifications carry little weight if their banking apps and websites cannot be relied upon.
"The Bank of England has recently announced it will begin to run IT stress tests on banks. These can't come soon enough."