Is there any need for the born approximation in spontaneous emission by a single two-level atom?

Summary In the case of spontaneous emission, up to date, it was believed that the Born approximation in the projection-operator method as well as in the Heisenberg equations of motion is inevitable. Therefore, no consistent comparison between these methods and that of Weisskopf-Wigner (where no such approximation has been used) was possible. This inconsistency is removed in the present paper.