Firm facing £500,000 fine over data leak

The list, produced by ACS:Law, which is targeting online pirates, was posted on the Internet following an attack on the firm's website.

It contained the names and addresses of more than 5,300 Sky broadband customers alleged to be illegally sharing adult films online.

Information Commissioner Christopher Graham told BBC Breakfast: "The question we will be asking is how secure was this information and how it was so easily accessed from outside.

"We'll be asking about the adequacy of encryption, the firewall, the training of staff and why that information was so public facing.

"The Information Commissioner has significant power to take action and I can levy fines of up to half a million pounds on companies that flout the (Data Protection Act)."

A spokesman for the Information Commissioner's Office (ICO) added that all breaches of the Act were taken "very seriously", adding: "The ICO will be contacting ACS:Law to establish further facts of the case and to identify what action, if any, needs to be taken."

Andrew Crossley, who runs ACS:Law, told BBC News: "We were the subject of a criminal attack to our systems. The business has and remains intact and is continuing to trade."