The influence of the flexoelectric effect on the phase diagram relevant to the order transition induced by an electric field on a nematic liquid crystal is considered. The analysis shows that this influence can be important. It is found that the order phase transition between the initially undistorted and distorted configurations always takes place. In contrast, the phase transition between the distorted and the saturated configurations is possible only if the dielectric anisotropy is large enough. The stability of the phases is analysed. The existence of a tricritical point is predicted. The dependence of the tricritical point on the flexoelectric coefficient is discussed too. The limits of our calculations and the performed simplifying hypotheses are critically analysed.