The effect of an applied external electric field on the binding energy of a donor impurity confined in a core/shell quantum dot is investigated taking into account the polarisation, self-polarisation contributions and the core/shell sizes. Calculations are performed in the framework of the effective mass approximation and using the Ritz's variational method. We show competition between the electric field and the polarisation. The advantage of our investigation is that it shows an interplay between a normal and an abnormal stark shift for certain regions characterised by typical core/shell sizes ratio. The dielectric mismatch effect due to the external medium dielectric constant has revealed an increasing of the binding energies.