HSBC is looking into payment problems thought to be linked to the national Bacs system.
Users complained on Twitter that payments were being held up, on a day when many employees would be expecting to receive their monthly salaries into their bank accounts.
Bacs processes transactions worth up to around £50 billion every day and is seen as critical to the functioning of the UK financial system.
HSBC said it is looking into problems. Bacs has yet to respond to the reports and it is unclear whether the problem was affecting only HSBC or was also hitting customers of other banks.
One Twitter user wrote: "None of my staff have been paid. HSBC when will this be sorted?" Another said: "Bacs payment system down on payday and before Bank Holiday. Lovely."
HSBC said on Twitter: "There has been an issue affecting Bacs payments by some HSBC customers today.
"We apologise for any inconvenience caused and are working hard to resolve the issue as quickly as possible."