About 1.2m people in the UK were affected by banking outages that happened on what was pay day for many earlier this year.
The details have emerged in letters from Lloyds, TSB, Nationwide and HSBC to Dame Meg Hillier, the chair of the Commons Treasury Committee, which is looking into the incident that occurred on Friday, 28 February.
HSBC also revealed that customers had to wait two hours on average that day to reach its online customer service team. Its standard target wait time is five minutes.
In their correspondence, the banks said they had paid compensation to affected customers and also outlined what they were doing to try to prevent similar problems in the future.