The company says they've recorded "suspicious activity" on a small proportion of the 9.5 million players registered online, indicating that people's login details, such as email and passwords, may have been stolen from other websites.
Camelot says hackers were not able to access "core National Lottery systems."
"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," Camelot said. "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."
Fewer than 50 accounts have been suspended since the cyber attack after their personal details were changed — though "some of these details may have been changed by the players themselves," Camelot said.
KEEP YOUR PASSWORDS STRONG AND VARIED
If your password is easy for you to remember, then it'll be easy for hackers, too. Try using symbols, numbers and capital letters throughout your passcode. Also, experts suggest you use different passwords for different accounts.
EMPLOY TWO-STEP AUTHENTICATION
Add another layer of security by having another code sent to your phone number before you can sign in.
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BEWARE OF PUBLIC WIFI
If you're traveling, verify with the coffee shop or hotel that the wi-fi name is valid -- many cybercriminals set up networks with similar names to popular spots. You can also set up a private VPN that encrypts all of your data that passes through the network.
COVER YOUR TRACKS
Wipe your hard drive clean before giving away, recycling or throwing out your old laptop or computer.
(Jonathan Kitchen via Getty Images)
DON'T LEAVE YOUR DEVICES UNATTENDED
That's just asking for trouble!
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BEWARE OF MYSTERIOUS URLS IN EMAILS
Don't ever click on URL from an unidentified or sketchy looking email.
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COVER YOUR WEBCAM
FBI director James Comey suggests placing a piece of tape over your webcam when you're not using it. If that doesn't convince you, note that Mark Zuckerberg is known to do the same.
KEEP YOUR SOFTWARE UP TO DATE
Hackers target vulnerabilities in software, which are often resolved in software updates, so stop hitting the "ignore" or "remind me later" button!
Discover More Like This
BACK TO SLIDE
The firm is contacting the owners of the accounts believed to have been compromised and instructing them to change their passwords. "We'd like to reassure our customers that protecting their personal data is of the utmost importance to us," Camelot's statement 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."