Banking group RBS lost £125 million as a result of the IT glitch that denied many customers access to their money for weeks earlier this year.

Most of the loss came from compensating customers, the bank revealed in its half-year financial results today.

There may be more costs in future, it added. "Additional costs may arise once all redress and business disruption items are clear and a further update will be given in Q3."

RBS gave some explanation of the incident, triggered by a failed update to its batch processing software CA7. "The immediate software issue was promptly identified and rectified," it said. "Despite this, significant manual intervention in a highly automated and complex batch processing environment was required. This resulted in a significant backlog of daily data and information processing.

"The consequential technology problems and backlog took time to resolve. However, at no point was any customer data lost or destroyed. Regrettably, in [RBS subsidiary] Ulster Bank, our customers experienced extended problems with their accounts, which have now been largely rectified."

RBS CEO Stephen Hester said the incident, which also cost him his bonus for the year, was a "significant blot" on the bank's reputation. "While we have significantly increased technology spend over the past three years, there is clearly more we need to do to ensure reliability for our customers," he said.

RBS is still conducting a "full and detailed investigation" of the causes of the problem.

Despite Hester's claim that the bank "significantly increased technology spend over the past three years", it has also cut over 1,000 IT jobs. In October 2011, an internal memo revealed that it had frozen IT spend for the year.

Overall, RBS made a £1.5 billion loss in the first half of 2012. Other losses included £135 million in compensation for customers who were mis-sold payment protection insurance and £50 million compensation for small businesses were mis-sold products to protect them against interest rate fluctuations.