making external framed and ledged doors internal!

like a fool i saw and bought 8 standard sized external framed and ledged doors to put in my about to be completed home. I have been told since that i will not be able to fit them into the already fitted standard internal door frames (even though they are the right size 6' 6" 1981mm by 2' 9" 838mm) are the doommongers correct or is there a way round this?
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