Consumers are being warned to be careful when buying second-hand Microsoft Xbox 360s because they may have been illegally modified to play pirated videogames.

The warnings being posted on forums follows news yesterday of the permanent ban imposed by Microsoft on modified consoles accessing the Xbox Live service.

There are an estimated 20 million people using the service to play games and it is believed that around one million modified consoles have already been barred.

The ban on these consoles is permanent so people who have modified their 360s will have to buy a new one if they wish to access the Xbox Live service in future.

However since news of the ban broke, forums are talking about advertising and listings sites such as Craigslist seeing the number of Xbox 360s for sale skyrocket.

Microsoft said in a statement: “Our commitment to combating piracy and support safer and more secure gameplay for the more than 20 million members of our Xbox LIVE community remains a top priority.

“All consumers should know that piracy is illegal and that modifying their Xbox 360 console to play pirated discs violates the Xbox LIVE terms of use, will void their warranty and result in a ban from Xbox LIVE.

"The health of the video game business depends on customers paying for the genuine products and services they receive from manufacturers, retailers, and the third parties that support them.

The consoles can still play games offline but if a person inadvertently buys a modified console they will not be able to access or use the Microsoft Live gaming service.

The move to stamp out the trade in chips that allow people to play pirated video games has moved up a notch recently. Also Microsoft makes it clear in its terms and conditions that it modifying an Xbox console will result in a ban.

“…any attempt to disassemble, decompile, create derivative works of, reverse engineer, modify, further sublicense, distribute, or use for other purposes the Service, any game, application, or other content available or accessible through the Service, or any hardware or software associated with the Service or with an original Xbox or Xbox 360 console is strictly prohibited and may result in cancellation of your account and/or your ability to access the Service, and the pursuit of other legal remedies by Microsoft."

Although Microsoft would not comment on how it could tell if a console had been modified, gaming experts have said that people wanting to play pirated video games using the service will simply try to find another way around the ban.