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 have said that after logging in, they have seen details of a different account holder.
NatWest said it was working with customers who had been affected and it was dealing with the issue.
One customer who could see the wrong details said the site closed down the view soon after the problem occurred.
“We are aware of an issue relating to Bankline. We are working hard to resolve this issue and are supporting customers who have been impacted,” said a spokeswoman for RBS, which owns NatWest.