Sony has delayed restoring its PlayStation Network gaming service.

The online service went down on April 20 following an attack, which resulted in thousands of personal details including credit card numbers and passwords falling into the hands of hackers.

Last week, Sony CEO and Chairman Howard Stringer said in an open letter: "In the coming days, we will restore service to the networks." However, Sony has since backtracked and decided to delay bringing the service back.

"We were unaware of the extent of the attack on Sony Online Entertainment servers, and we are taking this opportunity to conduct further testing," Sony said in a blog.

"We won't restore the services until we can test the system's strength in these respects."

In the letter to the US House of Representatives last week, Sony suggested hacking organisation Anonymous were behind the attack. Anonymous previously declared war on Sony when it sued 'geohot', the hacker George Hotz who released cracking codes for the PS3.

"We discovered that the intruders had planted a file on one of our Sony Online Entertainment servers named 'Anonymous' with the words 'We are Legion'," Sony said in a blog. 'We are Legion' is a phrase popularly associated with Anonymous.

However, in an interview with, Anonymous spokesman Barrett Brown has denied the group was involved in the incident.

"Anonymous has no record of engaging in credit card theft and resale, and if we did, the FBI would've already come down on us," he stated.

Sony has also admitted around 2,500 names and partial mailing addresses belonging to some PlayStation Network customers had been leaked on the web.

"The website was out of date and inactive when discovered as part of the continued attacks on Sony," the company said.

Sony revealed the site has since been taken down, and the majority of the data related to US users that entered a competition in 2001.