We consider the critical behaviour of binary mixtures described by the ϕ 4 Landau-Ginzburg free energy and subject to an external shear flow. The critical temperature is calculated as a function of the shear rate γ in the limit of an infinite number of components of the field. The two cases with conserved and non-conserved order parameter are analysed. The values of the critical temperature depend on the dynamics considered. In particular, at small γ , the critical temperature grows as γ 1/4 in the conserved case and as γ 1/2 in the non-conserved case.