The Sam Mobile site claims to have found version 'I9300XXDLI5' of the Galaxy S3 Jelly Bean firmware, which it says is "very much near perfect".
It estimates that Samsung will start rolling out the official Jelly Bean upgrade for the Samsung Galaxy S3 "very soon".
This would represent a much quicker turnaround than for Android 4.0 Ice Cream Sandwich, which took around six months to arrive on the Samsung Galaxy S2 after its launch last autumn.
Google announced the Android Jelly Bean operating system in June, and then launched the software on its Nexus 7 tablet computer a month later, and selected smartphones including the Samsung Galaxy Nexus.
Jelly Bean offers various improvements on its predecessor, including faster browsing via 'Project Butter' and a smoother user interface, along with fixes to the maps' compass and direction arrow.
There have already been several leaked versions of the Galaxy S3 firmware update, and it is thought that Samsung is letting them slip out to aid testing of the new platform.
Samsung has not yet given an official confirmation of the upgrade. But Sam Mobile notes: "We are getting leaks almost everyday so we believe that the final Official Jelly Bean update will be released in the next week."