Bank of Ireland has made a blunder on thousands of mortgage accounts it took over from KBC Bank.

The error is likely to mean that some former KBC customers will be turned down for loans while the issue is being resolved.

Around 35,000 mortgage holders are affected, which is around half of the number bought from KBC.

Bank of Ireland has written to mortgage holders who transferred to it from KBC telling them the error it made was “at the point your mortgage loan transferred from KBC to BoI which resulted in your mortgage ‘start date’ being incorrectly input into our systems”.

The letter says: “As we were aware of this error, we did not submit the mortgage details to the CCR [Central Credit Register]…We will continue to withhold monthly reporting until the error is fixed.”

The bank admitted that if a customer applies for a loan, or another form of credit, the gap in the monthly updates to the Central Bank’s Central Credit Register may have been seen by another lender.

This could lead to a refusal of credit for the likes of a car loan or other products for the former KBC mortgage customers.

The CCR helps a lender to decide whether it should approve an application for a loan or not, as well as giving the Central Bank better insights into patterns of lending in the broader economy.

Bank of Ireland sets out options in the letter for customers who have a credit application refused.

Asked about the error, BoI said: “The mortgage ‘start date’ for a number of mortgages transferring from KBC to Bank of Ireland was incorrectly inputted into our systems.

“As we were aware of this error, we did not submit the mortgage details to the CCR for February and March while it is being fixed. This was to prevent incorrect credit information being reflected on the customers’ credit report.”

The bank said that if a customer applied for a loan or another form of credit, the gap in the monthly updates to the CCR may have been seen by a lender. The bank said customers can use its letter as proof to other lenders of the error .

“To date we’ve had no confirmation of a loan application being impacted but if any customer has a specific query they can contact our team and we will support. We have also informed other lenders about the error so that they are aware of it.

“We are updating the records as a priority and will write to customers again when it is resolved, which is expected in the next few weeks.”

This month last year Bank of Ireland was fined €463,000 by the Data Protection Commissioner for messing up customers’ account details that could have affected their credit ratings.

The fine was also based on the bank not telling customers about the issue on time.

DPC Commissioner Helen Dixon ordered the bank to fix its sub-par data processing systems.

The data breaches involved, which occurred between 2018 and 2019, relate to cases where the bank submitted incorrect information to the Central Credit Register (CCR).

Last November it emerged that KBC had overcharged some mortgage customers who switched to another lender on break fees. It has now paid the affected customers compensation.

The miscalculation of break fees may have discouraged hundreds of people from switching from the bank ahead of the sale of the mortgages to Bank of Ireland.

Some KBC mortgage holders had been anxious to choose their own lender ahead of their home loans being sold to Bank of Ireland.

KBC Bank Ireland admitted it miscalculated the break fee for some of its customers who wanted to come out of a fixed rate and switch to another lender.