The UK's data protection watchdog has fined Sony for the PlayStation Network data breach in April 2011, which compromised millions of users' personal details.

The Information Commissioner's Office (ICO) has slapped Sony with a £250,000 (about $400,000) fine for the breach, arguing it was "a serious breach of the Data Protection Act."

The PlayStation Network hack in 2011 compromised the names, postal addresses, e-mail addresses, date of birth, and account passwords of millions of users. According to the ICO, customers' payment card details were at risk as well.

After conducting an investigation on the matter, the ICO concluded that Sony could have prevented the hack if its software had been up to date. Moreover, "technical developments" also put passwords' security at risk. The ICO has the authority to fine companies if there is a serious breach of section 4(4) of the Data Protection Act (DPA).

"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," said David Smith, Deputy Commissioner and Director of Data Protection. "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 distinguishing 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."

"This 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," added Smith, explaining why the penalty on Sony was so "substantial."

Meanwhile, Sony Computer Entertainment Europe told TechCrunch that it was the victim of a "focused and determined criminal attack," and no evidence indicated that users' encrypted card details were accessed at any time during the data breach. The company plans to appeal the ICO ruling.

Sony further added that it is constantly working on strengthening its systems, adding multiple layers of defense and struggling to make its networks "safe, secure and resilient," but criminal attacks on electronic networks are a growing trend of the twenty first century.