UK bank Abbey said an IT upgrade that caused problems for customers this week is related to an ongoing transformational programme.
Some Abbey small business customers were not able to see or make transactions on certain accounts as systems were upgraded.
"As part of Abbey's ongoing transformational programme, we have been upgrading our systems to provide our customers with better services and facilities," said an Abbey statement.
"As part of the upgrade, our active online customers have been required to register again to access to their secure internet site. The majority have done so successfully - however, for a small number of customers, for instance, those with linked accounts, if they have only re-registered for their main account, they are not able to see or transact on the other accounts," it said.
The bank is currently involved in a transformational project to migrate IT systems to the Partenon global banking platform of its parent, which has caused some problems for customers in the past. In November last year services to customers were disrupted as it grappled with migrating to the new banking system.
The project moves the bank from running separate systems that support different parts of the business to a single platform that can collate all the data about each customer. Once complete, the upgrade that caused the problems this week will enable "businesses to view all of their accounts via a single log-on", according to Abbey. The bank said the systems are now getting back to normal and added that "noone will be financially impacted by this".
During the Partenon project the bank moved its 10 million savings accounts, four million current accounts and eight million card accounts to the new platform.
The bank has renewed its branch communications network by building more than 45 portals for 26,000 employees and third-party organisations, and has also created a back-up datacentre.