Customers of a string of major banks have found themselves locked out of mobile banking amid high surges of traffic as people checked their pay.
Royal Bank of Scotland (RBS), which includes NatWest among its brands, said it had seen "record" numbers of people trying to access its app in the run-up to the weekend, as many tend to have their wages paid in around this time of the month.
Barclays and Santander also reported problems with mobile banking.
All the banks said that their other services were working as normal.
Customers vented their annoyance on Twitter.
One frustrated Barclays customer said: "Whhhhyyyy isn't the #barclays app working. I have no idea if I've been paid or not. It's driving me mental."
Another wrote: "PAYDAY! The one day I want to use your app and it's not working! Sort it out Barclays!"
A NatWest customer wrote "natwest app isn't working! It's either not even loading or taking forever to log me in!"
RBS apologised to customers on Twitter, saying that "high traffic" was preventing some customers from being able to log in.
It said: "We are currently experiencing record usage of our mobile banking app. Over 5,500 customers are logging on every minute."
NatWest tweeted to customers: "If you're having difficulty with our mobile app, online banking, telephone banking and ATMs are all working normally."
Barclays said it had also suffered problems but added that people were gradually managing to get back on to the app.
Santander said that a problem caused its mobile banking app to "stall" and the service was now back up and running.
A spokeswoman for Santander said: "We apologise for any inconvenience caused while we worked as fast possible to fix the problem.
"Customers were still able to transact using Santander telephone banking, Santander branches and online banking during this time."
RBS suffered hitches in December, when its websites were the victim of a cyber attack that left some customers unable to access their accounts in the group's second computer crash in less than a week.
That problem came just days after an embarrassing IT failure for the group on the busiest online shopping day of the year.
The RBS banking group announced this week that it is to ban teaser rates and "sweeteners" on its products in a bid to get back to basics and put customers at the heart of its business.
Barclays later confirmed that its problems had been fully fixed. The end-of-month demand on the system had meant that a technical problem had taken longer to resolve, although the initial cause was not due to higher traffic volumes.
A Barclays' spokesman said: "We apologise to our customers that have not been able to access the Barclays Mobile Banking system today.
"This was due to a technical incident that occurred this morning which has taken longer than we would have liked to be resolved. All other services, including online banking and Pingit continued to work as normal."