Sony has been hit with a £250,000 fine by the Information Commissioner's Office (ICO) for a breach of its systems in 2011.
The attack left millions of customers' detail exposed, including their addresses, email addresses, dates of birth and account passwords. The ICO said customers' payment card details were also at risk.
The data watchdog said the attack could have been prevented had Sony been using up-to-date software. It also criticised the password techniques used by the firm to protect users' log-in credentials.
Although the breach was a result of a deliberate attack by hackers, the ICO's deputy commissioner David Smith said Sony, as a leading technology company, should have been better prepared.
"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," he said.
Smith also said the ICO had taken such a tough line stance with the size of the penalty as a warning to firms of all sizes that keeping data secure is a top priority.
"The penalty we've issued today is clearly substantial, but we make no apologies for that. The case is one of the most serious ever reported to us. It directly affected a huge number of consumers, and at the very least put them at risk of identity theft," he 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."
However, Sony said it intends to appeal the ruling, noting that, as the ICO itself has admitted, the data loss was the result of a "focused and determined criminal attack".
"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," it added.
The fine is one of the largest ever levied against a private sector firm, with the public sector bearing the brunt of fines to date.
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