Internal shield in the rear fuselage of an aircraft having a propulsion system formed by two engines mounted on each side of it; the rear fuselage having at least a vertical symmetry plane; the rear fuselage being made of a composite material; the internal shield being located in said vertical symmetry plane and extended in an area that covers the possible trajectories of a set of pre-defined fragments detached from one of said engines in a failure event that would impact on critical elements of the opposite engine; the internal shield having a flat shape and an energy absorption capability that allows stopping said fragments. The invention also refers to a method for determining the area of an internal shield and to an aircraft having said internal shield.