Apple has warned that unlocked iPhones may become inoperable.

"Apple has discovered that many of the unauthorized iPhone unlocking programs available on the internet cause irreparable damage to the iPhone's software, which will likely result in the modified iPhone becoming permanently inoperable when a future Apple-supplied iPhone software update is installed," the company said in a statement.

Unlocking the iPhone is a process that hackers have developed to allow the device to work with SIM (Subscriber Identity Module) cards from carriers other than AT&T Wireless, Apple's preferred carrier. The applications and instructions developed by hackers over the past couple of months are freely available on the Internet, but users may wish they waited.

The first hint that Apple would take a more aggressive stand toward unlocking hacks came last week during the company's London press event to announce the UK ship date for the iPhone. Asked by a reporter what Apple was doing to stop people from unlocking their phones, CEO Steve Jobs called it "a cat and mouse game."

"I'm not sure if we are the cat or the mouse. People will try to break in, and it's our job to stop them breaking in," he added.

Apple made it very clear that it would not be responsible for any damage caused by the installation of third-party unlocking software.

"Users who make unauthorized modifications to the software on their iPhone violate their iPhone software license agreement and void their warranty," the company said in its statement. "The permanent inability to use an iPhone due to installing unlocking software is not covered under the iPhone's warranty.

The company said that the next software update for the iPhone would be available later this week. That update will include the recently announced iTunes WiFi store.