By Marcus Leach

Technology giants Sony have laid the blame for their mass security breach at the feet of online vigilante group Anonymous.

The breach allowed a hacker to gain access to the personal data of over 100 million of Sony's online gamers.

Sony, in a letter to the US Congress, said the breach of security came at the time when the were fighting a denial-of-service attack from Anonymous. Such attacks take servers down by overloading them with traffic.

However, Anonymous have denied being involved in data theft, with Sony claiming the group had acted in revenge due to the federal court action the company took against a hacker in San Francisco.

Sony went on to say that the theft of data came from a separate attack, whilst they were distracted with the denial-of-service attack, although they were unsure if the two attackers were working in unison.

"Whether those who participated in the denial of service attacks were conspirators or whether they were simply duped into providing cover for a very clever thief, we may never know," Sony's letter said.

Sony revealed that they had, on Sunday, discovered a file planted on one of its servers named Anonymous and featuring the line "We are legion", which is a phrase used by the group.

In the letter to members of the House Commerce Committee, Kazuo Hirai, chairman of Sony Computer Entertainment America, defended the way that his company had dealt with the breach.

Questions had been raised as to the manner in which Sony dealt with the breach of security. Sony discovered the breach in its Playstation video game network on 20 April but did not report it to US authorities for two days and only informed consumers on 26 April.

"Throughout the process, Sony Network Entertainment America was very concerned that announcing partial or tentative information to consumers could cause confusion and lead them to take unnecessary actions if the information was not fully corroborated by forensic evidence," the letter said.