The claim was made in The Sun and alleged that, in a leaked memo, the casting company for the ad campaign, Capataz Casting, had been told that actors put forward should not be "fat or overweight".
McDonald's has insisted this specification did not appear on the original casting brief.
In a statement, it said: "We are both surprised and shocked that our original approved casting brief has appeared 'embellished' in this inappropriate way and we are currently investing how this happened during the agency cascade process."
The statement adds that a full review of McDonald's production processes with its agencies will now be launched.
Capataz Casting head Simon Allen said: "I think there's been a misunderstanding over interpretation in terms of the brief," but declined to comment further.
It is reported that this latest advertising push is set to feature a "dull regular guy" who transforms into a superhero.
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