Internet activist group Anonymous has denied Sony's accusation that it allowed the personal details of 100 million customers to be hacked.

It refuted insinuations which were made in a letter sent by Sony to American Congress officials linking the group to last month's security breach which involved PlayStation Network, Quiocity and Sony Online Entertainment accounts.

Anonymous, which became well-known following Wikileaks-related attacks, released a 900-word statement denying involvement.

The group stressed that its aims were purely politically and not financially motivated, responding to news that credit card details were among a string of personal data stolen.

Hackers who breached Sony's systems also accessed names, addresses, countries, email addresses, birth dates, usernames, passwords and online handles.

It led to the Japanese electronics company shutting down its online services to allow investigators to discover the source of the breach and bolster security.

Sony told officials yesterday it "discovered that the intruders had planted a file on one of our Sony Online Entertainment servers named 'Anonymous' with the words 'We are Legion'".

Anonymous uses those three words as part of its slogan.

Kazuo Hirai, chairman of Sony in America, believes the attack on its services to have been carefully planned, calling it a "very professional, highly sophisticated criminal cyber attack."

But the statement by Anonymous distanced itself from the breach, saying: "Anonymous has never been known to have engaged in credit card theft". The group added: "Public support is not gained by stealing credit card info and personal identities, we are trying to fight criminal activities by corporations and governments, not steal credit cards."

It added: "If a legitimate and honest investigation into the credit card is conducted, Anonymous will not be found liable. While we are a distributed and decentralized group, our 'leadership' does not condone credit card theft.

"We are concerned with erosion of privacy and fair use, the spread of corporate feudalism, the abuse of power and the justifications of executives and leaders who believe themselves immune personally and financially for the actions they undertake in the name of corporations and public office."

Sony also accused Anonymous of distracting its engineers by organising a denial-of-service attack which allowed the hackers to steal data undetected but there was no mention of this in Anonymous' response.

Anonoymous has been involved in a number of denial-of-service attacks on high profile companies including Mastercard and Visa, which was in response to both firms withdrawing support for Wikileaks, which had begun publishing 250,000 leaked diplomatic cables.

Such attacks work by bombarding sites until they crumble under the pressure.