Customers of NatWest's online business banking service have seen other companies' balances and transactions after logging in.
Bankline allows online management of accounts and says on its website that it offers "security you can trust".
But customers said that after logging in, they had seen details of a different account holder.
NatWest said it was an isolated incident affecting a small number of customers and had now been resolved.
It said it was working with customers who had been affected in order to keep them up to date and provide support.
Currently logged in to @NatWest_Help Bankline and can see a completely different company's bank details, balances and transactions. Clearly something has gone VERY VERY wrong.— Alex (@MetAlbertR) September 26, 2019