Thousands of customer accounts on the National Lottery website may have been compromised.
Camelot said it believes that "around 26,500 players' accounts were accessed", but fewer than 50 accounts have had activity take place since the hack.
The National Lottery operator said it became aware of "suspicious activity" on a number of players' online National Lottery Accounts on Monday.
Camelot said: "Of our 9.5 million registered online players, we believe that around 26,500 players' accounts were accessed.
"A much smaller number - fewer than 50 - have had some activity take place within the account since it was accessed. This was limited to some of their personal details being changed - and some of these details may have been changed by the players themselves.
"However, we have taken the measure of suspending the accounts of these players and are in the process of contacting them to help them re-activate their accounts securely.
"In addition, we have instigated a compulsory password reset on the accounts of the 26,500 affected players. We are in the process of pro-actively contacting them to help them change their passwords, as well as giving them some more general online security advice."
Camelot said it wanted to make clear that there has been no unauthorised access to core National Lottery systems or any of its databases, which would affect National Lottery draws or payment of prizes.
"In addition, no money has been deposited or withdrawn from affected player accounts," the statement added.
Camelot said it believes 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," the operator said.