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The trojan contains false installation information that causes other applications to be removed when the trojan is uninstalled, the firm says. The threat was first reported on ModMyiPhone.

F-Secure

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says this discovery should alert iPhone owners to the dangers of unlocking iPhones using a method that exploits a security flaw.

"Hopefully this serves as a warning for those who have opened their iPhones using a security hole in the system and then installing unverified software without a second thought to what they are doing," says Jarno Niemela on the F-Secure website.

"This time it was an 11-year-old kid playing with XML files who created the trojan. Next time it might be someone else with more skills and with a specific target," Niemela warns.

A popular iPhone hack relied on a vulnerability in the handling of TIFF images, which Apple soon fixed. However many users will not have applied Apple's patch, knowing that it is likely to relock their phone.