Millions of PlayStation users have been warned their personal information, including credit card details, may have been stolen.
Gamers have been locked out of the network for a week, but owner Sony revealed yesterday the system has been down since it was hacked last Wednesday.
There are three million users in Britain and 70 million worldwide.
PlayStation members are required to submit credit card and personal details to play online games and download software, films and music.
Should I be worried about my card details?
Financial Fraud Action (FFA), which represents card firms, says Sony is due to pass it the details of all card numbers that may have been stolen, which FFA will then distribute to banks and building societies.
FFA says this is standard procedure after a hack.
It therefore says anyone who has entered their card number on the PlayStation network does not need to contact their provider as firms will cancel many cards automatically.
FFA adds in a statement: "Customers should keep a close eye on their account for any unusual activity – if they spot any they should contact their bank or card company.
"If anyone is the innocent victim of fraud they will get their money back from their bank or card company."
Sony is not certain that credit card details have been stolen but states on its website: "While there is no evidence credit card data was taken we cannot rule out the possibility.
"To be on the safe side we are advising your credit card number (excluding security code) and expiration date may also have been obtained."
What about my passwords?
Names, addresses, dates of birth, PlayStation log-in details and email addresses are also thought to have been taken by the hackers.
FFA is therefore also advising users to change passwords for all their accounts, be it a bank or email account, if it is the same as they use on PlayStation.
And what about ID fraud?
Playstation users also need to be alert to the threat of identity theft, according to credit reference agency Equifax.
It says fraudsters only need three items of personal information to be able to steal an individual's identity.
If you're worried, you can check your credit report to identify whether fraudsters try to open accounts in your name (see the Credit Rating guide to check for free).
Josh Shaul, chief technology officer for Application Security Inc., which makes security software, called the intrusion "one of the worst breaches we've seen in several years".
If credit card information was stolen, the heist would rank among the largest known thefts of financial data.
Sony says it discovered that between 17 and 19 April, there was an "illegal and unauthorised" intrusion of its network and warned users to look out for telephone and email scams.
The PlayStation Network posted an apology to users through the Sony website saying it would email those suspected to be hacking victims.
It says: "We don't have an exact date to share at this moment as to when we will have the services turned on, but are working day and night to ensure it is as quickly as possible.
"Please note that we are as upset as you are regarding this attack and are going to proceed aggressively to track down those responsible."
Additional reporting by the Press Association.