TalkTalk has admitted the personal details of 156,959 customers and 15,656 bank account numbers and sort codes had been accessed in last month's cyber attack.
But the communications firm has stressed that the "information accessed cannot on its own lead to financial loss".
The telecoms company previously said the attack was on its website rather than its "core systems".
It now says 28,000 obscured credit and debit card numbers that were accessed cannot be used for financial transactions, and were ‘orphaned’, meaning that customers cannot be identified by the stolen data.
Personal details accessed include: name, address, date of birth, telephone number and email address. TalkTalk My Account passwords were not accessed,
"Our ongoing forensic analysis of the site confirms that the scale of the attack was much more limited than initially suspected, and we can confirm that only 4% of TalkTalk customers have any sensitive personal data at risk," the firm said. "However, we continue to advise customers to be vigilant, and to take all precautions possible to protect themselves from scam phone calls and emails.
"It was a difficult decision to notify all our customers of the risk before we could establish the real extent of any data loss. We believe we had a responsibility to warn customers ahead of having the clarity we are finally able to give today.
"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."
The company told customers it will not call or otherwise contact them regarding the incident and ask for bank details or other financial or personal information.
And it stressed it does not store complete credit and debit card details on the website. All card details had a series of numbers hidden, TalkTalk said, and therefore were not usable for financial transactions e.g. 012345 xxxxxx 6789
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