In a statement the company says that unlocking the phone so that it may be used on any network could turn it into a very expensive paperweight.
"Apple has discovered that many of the unauthorised 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 statement says.
It also warns that should the phone become unusable, repair will not be covered by any guarantee.
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"Users who make unauthorised modifications to the software on their iPhone violate their iPhone software license agreement and void their warranty," it says. "The permanent inability to use an iPhone due to installing unlocking software is not covered under the iPhone's warranty."
Apple said that it plans to release the next iPhone software update, containing new features including the iTunes Wi-Fi Music Store, later this week.
The iPhone goes on sale in the UK on 9 November and will be locked to O2's network.
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