NatWest customers faced more frustration after they found themselves locked out of the bank's mobile app.
Around two million people use the service and the problem has also affected customers of Royal Bank of Scotland (RBS) and Ulster Bank, which are part of the same group.
All the banks belong to RBS Group, which apologised to customers and said it was working to fix the problems which have prevented people from logging in.
The hitch is thought to have started at around 7am on Thursday and is the latest technical problem to have beset the group, starting with a huge IT meltdown last summer.
A spokeswoman for RBS could not say how many customers have been affected, although both personal and business users have been hit by the problems, which have struck just before the busy Easter weekend as people make plans to get away.
There are more than two million active users of the app among RBS and NatWest personal banking customers alone, with 13 million log-ins taking place each week.
The latest IT issues come just three weeks after a hardware fault prevented RBS Group customers from using cash machines and also affected online and telephone banking services.
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 bulk of the redress costs related to Ulster Bank, where the problems took weeks longer to clear up.
A statement from RBS Group said: "We are aware of a technical problem this morning which is preventing customers from logging in to our mobile banking applications. We are working to fix the problem and apologise to customers for the inconvenience caused. No other systems are affected."
The collapse also comes just weeks after the bank unveiled a new mobile banking feature for RBS/NatWest customers called Pay Your Contacts, which allows them to send payments of up to £100 to anyone with a Visa card, by entering their mobile number.
Most complained about financial products
Bank customers locked out of app
Figures from charity Age UK show that 29% of those over 60 feel uncertain or negative about their current financial situation - with millions facing poverty and hardship.
Even though saving for retirement is not much fun, the message is therefore that having to rely on dwindling state benefits in retirement is even less so.
To avoid ending up in this situation, adviser Hargreaves Lansdown recommends saving a proportion of your salary equal to half your age at the time of starting a pension.
In other words, if you are 30 when you start a pension, you should put in 15% throughout your working life. If you start at 24, saving 12% of your salary a year should produce a similar return.