Around 275,000 payments likely to total hundreds of millions of pounds have been held up by a glitch in HSBC's systems.
The fault in the payments, which has affected many people due to receive their salaries for the month, relates to the bank's business customers.
HSBC said it was working to resolve the problem but it still did not know whether it would be able to process the payments by the end of the day.
A spokesman said: "There has been a fault in the information used to process some payments from HSBC business customers.
"Approximately 275,000 payments have been affected, including payments to customers of other banks.
"HSBC apologises for the inconvenience this has caused. We are taking immediate steps to ensure the payments reach beneficiaries as quickly as possible.
"We will work with other banks to ensure that customers do not lose out as a result of today's problems."