A new laminar flamelet model is presented for Large Eddy Simulation ( LES) of premixed turbulent combustion. The model uses a marker field S+ to capture the flame. Compared to previous work on marker fields, a modification was introduced to simplify the boundary conditions. Although the scalar S+ is smooth in space, it allows describing steep flame fronts. The S+ equation captures the laminar or turbulent flame propagation via a reactive diffusive balance. The present LES model was used to simulate a turbulent premixed flame stabilized behind a sudden expansion. The LES predictions agree well with available experimental data in term of reproducing the time averaged velocity field as well as for capturing the flame dynamics and flame response to a perturbation.