Sony has been fined £250,000 for a breach that compromised the personal information of millions of PlayStation users.
The Information Commissioner's Office (ICO) issued the penalty after it found the attack on the Sony PlayStation Network in April 2011 could have been prevented.
Personal information including customers' payment card details, names, addresses, email addresses, dates of birth and account passwords were exposed.
David Smith, ICO deputy commissioner and director of data protection, said: "If you are responsible for so many payment card details and log-in details, then keeping that personal data secure has to be your priority.
In this case that just didn't happen, and when the database was targeted - albeit in a determined criminal attack - the security measures in place were simply not good enough.
"There's no disguising that this is a business that should have known better. It is a company that trades on its technical expertise, and there's no doubt in my mind that they had access to both the technical knowledge and the resources to keep this information safe."
The ICO added that following the breach, Sony rebuilt its Network Platform to ensure that the personal information it processes is kept secure.
Sony temporarily shut down its network, a system that links gamers worldwide in live play, after discovering the massive security breach.
The company said at the time that personal data from 77 million users of Sony's PlayStation network had been compromised. Users were all asked to change their passwords following the breach.
Sony faced heavy criticism over its handling of the network intrusion, as it did not notify consumers of the breach until a week after it began investigating unusual activity.
A Sony spokesman said the firm will be appealing against the ICO's decision.
He said: "Sony Computer Entertainment Europe (SCEE) strongly disagrees with the ICO's ruling and is planning an appeal.
"SCEE notes, however, that the ICO recognises Sony was the victim of 'a focused and determined criminal attack', that 'there is no evidence that encrypted payment card details were accessed', and that 'personal data is unlikely to have been used for fraudulent purposes' following the attack on the PlayStation network.
"Criminal attacks on electronic networks are a real and growing aspect of 21st century life and Sony continually works to strengthen our systems, building in multiple layers of defence and working to make our networks safe, secure and resilient."
He added that the security of customers' data is "of the utmost importance" to Sony.