HSBC admitted it has not informed customers of the change in policy, which was implemented in November.
The bank says it has now changed its guidance to staff.New rules
Stephen Cotton went to his local HSBC branch this month to withdraw £7,000 from his instant access savings account to pay back a loan from his mother.
A year before, he had withdrawn a larger sum in cash from HSBC without a problem.
But this time it was different, as he told Money Box: "When we presented them with the withdrawal slip, they declined to give us the money because we could not provide them with a satisfactory explanation for what the money was for. They wanted a letter from the person involved."