Current account customers with Nationwide, the UK's largest building society, have not received their December pay because of an "overnight computer error".

A processing problem has meant that customers with a Flex current account have not received payments that were made to their accounts last night. The glitch affects all Flex account customers, but the mutual said that only a "small number" of customers have been left without cash for New Year's Eve.

This morning Nationwide assured anxious customers that the problem would be resolved by midday but customers are still waiting to receive their income for this month. The building society has said that the problem will be resolved by the end of the day.

One Flex account customer, who asked not to be named, told The Times that he felt extremely worried when he checked his account this morning. He said: "It has been very inconvenient and I'm disappointed it has taken Nationwide such a long time to resolve. I have my bills coming out tomorrow so I just hope that the problem is sorted quickly."

Customers have been told to go into branches with evidence of their monthly salaries, such as payslips and monthly statements, for a cash advance. Nationwide has also promised that any interest due to customers will be refunded and it will also reimburse customers who have incurred any charges because of the computer error.

A spokeswoman for Nationwide said: "We have had an overnight problem, which has meant that a small number of customers have been paid but the money has not appeared in their accounts. We are working as hard as we can to resolve the problem."

Apacs, the UK payment association, has confirmed that the problem is with Nationwide, rather than the BACS system, which is used to pay millions of salaries each month.