Around 21,000 unique bank account numbers and sort codes were accessed in the TalkTalk cyber attack, the firm said on Friday.
ITV News understands the bank details were unencrypted.
The firm insisted the data hacked was "significantly less than originally suspected".
Earlier, police confirmed that a second boy has been arrested in connection with the alleged attack.
North Antrim MP Ian Paisley has appealed to the press on behalf of the family of a 15-year-old boy allegedly involved in the TalkTalk hacking case to respect their request for privacy.
As TalkTalk finally confirms the real extent of its data breach, it's time for the company to be more open with customers.
Around 21,000 unique bank account numbers and sort codes were accessed in the TalkTalk cyber attack, the firm said on Friday.
Police have made a second arrest, a 16-year-old boy from Feltham, in connection with the investigation into alleged TalkTalk data theft.