BT customer accounts have been debited without warning after the telco suffered a technical problem with a server.
Rules set by banks state that direct debits of variable amounts must be available to customers to view before the money is taken out of their account.
But BT confirmed that a technical fault on its online billing system meant that some customers who receive their BT bill online were not able to view them in advance of payment. The problem occurred throughout October.
"For a small number of customers, we have collected the correct amount of the bill, on the correct date, but without customers being able to view their bill online," said BT.
David Grey, a BT customer in Hampshire, said had money taken out of his account without warning. "The debit hit the bank account and there was no notice."
A spokeswoman at UK payment association Apacs said, "The question is what steps has BT taken to inform customers of the problem?
"Because of a lack of advanced warning individual customers might not have been able to make sure they had enough funds," she said.
BT said affected customers are covered by the Direct Debit Guarantee scheme.
These guarantees state that if the amounts to be paid or the payment dates change the company collecting the payment will notify customers in advance of an account being debited. If errors are made customers are entitled to a full refund.
The problem would be sorted out within a few days, BT said.