One could sport a 4.7-inch screen and the other a 5.7-incher, Reuters said, citing Asian supply chain sources.
Apple's move appears to have been spurred by the unexpected success of Samsung's Note 2 "phablets" which boast 5.5-inch displays. Samsung is said to be planning to launch a Note III with a 6-inch display.
Most reports have tipped the Note 3 to take its official bow at the annual IFA event in Berlin late this year. But some recent reports are suggesting it could make an early appearance at a press event in London soon.