There have been several instances of piracy on the Mac App Store already, just hours after it was launched, it has been claimed.

According to a report on Apple Insider, it took a matter of hours for pirates to figure out how to install and run unauthorised versions of paid-for apps.

By exchanging the receipt and signature files for certain paid-for apps - which can be downloaded from third-party websites, according to the report - with the receipt copied from a free app, you can in some instances get the unauthorised app to run.

Apparently, this is only possible with apps that haven't followed Apple's official app validation advice. John Gruber of Daring Fireball said: "It appears that many apps don't perform any validation whatsoever, or do so incorrectly. Apple should test for this in the review process, and reject paid apps that are susceptible to this simple technique."

A separate report on Gizmodo this week stated that a software packaged called Kickback - which won't be released until next month - would make it possible to pirate any application in the Mac App Store.

Kickback is apparently the work of Hackulous, and more details of how it works can be found on the Gadgets DNA website. Hackulous is the group that cracked Apple's DRM (digital rights management) system for iOS.

If the reports are correct, they add to a growing list of concerns about the Mac App Store. Developers have already expressed doubts about pricing, a lack of consumer choice and technical support "disasters", as Macworld reported earlier this week.