Bank of Scotland (BoS) online banking customers have been unable to access all their recent transaction details after a weekend system upgrade by parent Lloyds Banking Group.
Lloyds is currently migrating Halifax and Bank of Scotland customers onto the Lloyds TSB IT platform, a move understood to have created the issue over the weekend.
The changes have led to customers only being able to see limited details of their accounts since yesterday morning.
Lloyds has not detailed exactly which software was affected, but said it was tackling the issue and will have restored all services in the next few hours.
A company spokesperson apologised for the problem and said: "We've made some changes to our online banking system this weekend. We're aware that there is currently an issue that means that customers cannot see a full history of their transactions when they log into their online banking."
Last month, Lloyds said that the migration of Halifax and Bank of Scotland retail and commercial customer accounts onto the Lloyds TSB IT platform was a "key final step" to completing the merger of the companies.
"The completion of integration moves the group to a single platform," it said, describing it as a "key enabler" in its transformation programme, under which it is attempting to slash £2 billion from costs.
Lloyds, which works with IT suppliers including SAP, VMware, Oracle, Microsoft and Unisys, said that in the first half of the year it also completed a programme to move Bank of Scotland cash machines on to the Lloyds TSB platform, rolled out an integrated Mortgage Sales Platform to Cheltenham & Gloucester, and created a single claims management system within its general insurance business.
The merger has also led to thousands of job cuts among back office staff and management.