The Natwest and RBS websites are down, leaving users unable to manage their account.
It is just the latest in a string of issues for both banks, which have been repeatedly hit by IT problems this year.
The banks said that users should get in touch directly if they have any immediate issues, and that they were working hard to fix their websites.
“Our website is currently unavailable. We’re working hard getting it back up and running for you,” Natwest wrote on Twitter. “We’ll post an update when we have more information. In the meantime, please use our mobile app or contact us if you need urgent help.”
Natwest and RBS have been hit by the most issues of any bank in the last three months, according to research by the BBC reported last week. NatWest had been hit by 25 different outages 12-months to the end of June, it said.
