Telecoms group TalkTalk (LON:TALK) says the cyber-attack on its web site exposed the personal details of 156,959 customers.
TalkTalk said in an update on Friday that the hackers gained access to 15,656 bank account numbers and sort codes.
But it said "only" 4% of TalkTalk customers had sensitive personal data at risk and the 28,000 obscured credit and debit card numbers exposed could not be used for financial transactions.
The raid sparked a police investigation and a number of arrests. This week police released a 16-year-old boy on bail who was the fourth person arrested in connection with the attack, the BBC reported.
TalkTalk said the scale of the raid was much more limited than initially thoughtm but it added: "We continue to advise customers to be vigilant, and to take all precautions possible to protect themselves from scam phone calls and emails.
"We have now contacted all customers who have had financial details accessed, reiterating our advice on what to do to keep themselves safe.
"The financial information accessed cannot on its own lead to financial loss. We will be contacting all other affected customers in the coming days."
Shares in TalkTalk rose 15.1p or 6.8% to 236p in early London trading - down from about 408.8p in June, their highest level this year.