NatWest online business banking customers said they could see other customers' balances and transactions.
NatWest online business banking customers complained they could see other customers' balances and transactions, despite the bank's pledge that it offers "security you can trust".
The glitch was flagged up a number of NatWest customers, who sounded off on Twitter, saying that after logging in, they had seen details of a different account holder.
Twitter user @MetAlbertR said: "Currently logged into @NatWest_Help Bankline and can see a completely different company's bank details, balances and transactions. Clearly something has gone very very wrong."
Another annoyed Twitter user @stba2006 raged: "The same just happened to us. We contacted NatWest who were totally disinterested."
Twitter user @tottway75 added: "I had the same. More worrying when you call to report it & are told that they are a 'bit' concerned with what is happening! Not good enough."
The bank's bankline service allows customers to manage their accounts online and maintains that it offers "security you can trust".
NatWest said the incidents were isolated, affecting a small number of customers and were soon resolved.
The bank said it was working with customers who had been affected to provide support and keep them updated.
The glitch follows just days after another hitch at the bank left customers locked out of accounts.
NatWest, which has more than 7.5m customers, updated its app- but the upgrade left a "small number" unable to access their accounts.
The issue also affected RBS—which has nearly 19m customers globally—also.
Customers who downloaded an update were told that version was no longer being supported.
NatWest said the error had only affected a small number of customers.