The bank has admitted that a processing error caused customers' accounts to be debited twice for money withdrawn from cash machines over the weekend.
The bank said it identified the error on 9 October and has now rectified the situation. Customer accounts have been automatically re-credited. The problem occurred on Monday night, while systems were processing cash that customers had withdrawn over the weekend.
It is not yet known what caused the problem, which one national newspaper claimed resulted in the removal of £30m of customer funds.
NatWest spokesman Nick Gill told CW360 that a batch tape of the weekend's transactions was accidentally processed twice. "I imagine it was a human error," he said. A thorough investigation is currently under way.
Despite the media speculation, Gill told CW360 that the bank does not know how much money was taken in error.
"There's no estimate of the value of the transactions," Gill continued. "We have to ensure that this doesn't happen again, but customers will not be affected and do not need to do anything."
While fears have been raised that customers could incur charges or have transactions refused as a result of the error, Gill reassured customers that any account charges would be "sorted".