Around 26,000 National Lottery accounts are feared to have been hacked.
The firm said it did not believe its own systems had been compromised, but that the customers’ login details had been stolen from elsewhere.
The company also added that no money had been taken or added to the compromised accounts.It did however say that there had been suspicious activity on fewer than 50 of them.
The Information Commissioner’s Office has launched an investigation into the matter.A spokeswoman said: "Camelot submitted a breach report to us last night which we have reviewed. We will be talking to Camelot today.
"The Data Protection Act requires organisations to do all they can to keep personal data secure - that includes protecting it from cyberattacks. Where we find this has not happened, we can take action.
"Organisations should be reminded that cybersecurity is a matter for the boardroom, not just the IT department."
Camelot, the firm’s operator, said it had become aware of the problem on Sunday.In a statement it said: "We are currently taking all the necessary steps to fully understand what has happened, but we believe that the email address and password used on the National Lottery website may have been stolen from another website where affected players use the same details.
"We do not hold full debit card or bank account details in National Lottery players' online accounts and no money has been taken or deposited.
"However, we do believe that this attack may have resulted in some of the personal information that the affected players hold in their online account being accessed.”
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