Customers of the telecoms company TalkTalk are being warned that their personal data could have been compromised following a “significant and sustained” cyber-attack.
The company said credit card and bank account details may have been accessed by internet hackers who targeted its website.
The Metropolitan Police have launched a criminal investigation into the attack on Wednesday.
TalkTalk was also investigating reports of customers’ accounts being targeted, although some incidents may not be linked to the cyber-attack.
The company said on its website: “We are investigating reports that customers’ bank accounts have been affected as a result of this week’s criminal attack, although at this stage there is no evidence that this is the case.
“We do know that there are a small number of customers who have previously been targeted by criminals and fallen victim to scams, and we are continuing to support those affected.”
Customers who believe they have been affected are advised to contact their bank and the internet crime agency Action Fraud on 0300 123 2040 or via www.actionfraud.police.uk
TalkTalk said: “That investigation is ongoing, but unfortunately there is a chance that some of the following data has been compromised: names, addresses, date of birth, phone numbers, email addresses, TalkTalk account information, credit card details and/or bank details.
“We are continuing to work with leading cyber crime specialists and the Metropolitan Police to establish exactly what happened and the extent of any information accessed.”
Police said enquiries were ongoing and no arrests had been made.
Det Supt Jayne Snelgrove, of the Metropolitan Police Cyber Crime Unit, said: “This case is just one example of the new generation of criminality my team are dedicated to tackling.
“We continue to lead on this investigation but are working with the National Crime Agency.”