This paper is concerned with an analysis of the derivation of a nonlinear evolution equation governing the dynamics of a weakly unstable perturbation in a mixing layer. It is demonstrated that the Landau constant has no universal character and is determined by the degree of supercriticality of perturbation, whose measure is represented by the linear growth rate of instability $\gamma$ (which is proportional to the departure from neutral wavenumber). It is shown that the interaction of the fundamental harmonic of the perturbation with the associated distortion of a mean flow is the dominant nonlinear effect. It is this that makes the main contribution to the Landau constant rather than the interaction with the second harmonic as was thought previously. We examine the regimes of both a viscous and a non-stationary critical layer and calculate the Landau constant. It is shown that, except for the case when the supercriticality is very small, $\gamma \simeq \nu$ ($\nu$ is the inverse of the Reynolds number), the nonlinearity cannot substantially influence the exponential growth of the perturbation predicted by linear theory. When $\gamma \simeq \nu$, however, the nonlinear time exceeds the time of viscous spreading of the initial flow and for a correct formulation of the problem, one has, as was first pointed out by Huerre, to introduce here an artificial force field.