The UK National Lottery operator Camelot has revealed that a "very small" number of its customer accounts have been accessed illegally, which may have allowed hackers to obtain users' personal information.

Camelot said Wednesday that its online security monitoring had identified around 26,500 accounts which were accessed by the hackers, a small percentage of its 9.5m registered online players.

Less than 50 accounts have been used since the hack, although Camelot says this could be a case of the original account holders changing their details. Nevertheless it has suspended these accounts and is in the process of contacting players to help them securely reactivate their service.

In addition the operator has initiated a compulsory password reset for the 26,500 affected players.

"We are in the process of proactively contacting them to help them change their passwords, as well as giving them some more general online security advice," said the national lottery operator.

Camelot stressed that there has been no unauthorised access of its core systems or databases which would affect National Lottery draws or the payment of prizes. In addition, no money has been deposited or withdrawn from the affected accounts, as they do not hold debit card or bank information.

Camelot believes that email addresses and passwords may have been stolen from another website for which players use the same details, allowing the hackers to access personal information.

"Cyber criminals such as this are persistent, and we are continuing to monitor and protect our systems," the operator said. "We are also working closely with the National Crime Agency and the National Cyber Security Centre on an ongoing basis on this criminal matter.

"We'd like to reassure our customers that protecting their personal data is of the utmost importance to us," Camelot added. "We are very sorry for any inconvenience this may cause to our players and would like to encourage those with any concerns to contact us directly, so we can discuss it with them in more detail."

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