Bank of Scotland has spoken out to reassure its customers that the near-total computer failure, which caused its systems to crash for three hours last week, will never happen again.
The breakdown, which occurred during the busy lunchtime period last Wednesday, threw the bank's infrastructure into chaos, affecting all 350 branches and crippling its network of more than 1,000 automated teller machines.
The system failure also brought down Bank of Scotland's Internet banking arm and prevented customers from making high-value purchases with Switch debit cards and the bank's phone lines were affected.
Although a full service was resumed late on Wednesday, the system failure caused a number of processing delays which meant transactions usually carried out overnight were held up.
A bank spokeswoman said, "The situation was resolved but we encountered a processing backlog which caused problems. Processing which should have been done overnight was causing some difficulty but by lunchtime on Friday, everything was resolved."
The system failure comes at an embarrassing time for the bank, which recently entered into a high-profile 10-year outsourcing deal with IBM.
Under the terms of the £700m deal, which was signed in July, IBM was to take over responsibility for the management and operation of all the administration, hardware, software, data-processing and infrastructure needs of the bank from 1 September.
Although Bank of Scotland refused to comment on the nature of the technical problems, it categorically denied the failure was anything to do with its recent outsourcing deal with IBM.
The spokesman said, "It has nothing to do with IBM whatsoever. We had a very rare and unusual system fault which affected all services for a short time but we are confident this particular problem will never be repeated again."
But a source close to Bank of Scotland said the problem was the result of a faulty piece of software, which had allowed two sets of data to converge at the same time, thereby causing the system to crash.
After the fault was detected the software was removed and was not replaced, the source said.